US3518622A - Expressway ramp traffic control system - Google Patents

Description

June 30, 1970 J. L. BARKER ET AL 3,518,622

EXPRESSWAY RAMP TRAFFIC CONTROL SYSTEM Filed April 14, 1967 4 Sheets-Sheet 1 illiliilj llll 5 r y was m fi M h h l mama m. V W a a M fi 1 ZMW/ M ma cuh W W. |l|.li

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EXPRESSWAY RAMP TRAFFIC CONTROL SYSTEM 4 Sheets-Shem Filed April 14, 1967 United States Patent US. Cl. 340-36 9 Claims ABSTRACT OF THE DISCLOSURE A system for controlling entrance of vehicles onto a limited-access roadway from an entrance ramp. The empirical capacity of the roadway is modified to reflect traffic conditions on the roadway. The traffic volume or demand upstream of the entrance ramp is subtracted from this modified capacity to give available capacity. When the time integral of the available capacity indicates that there is sufficient room on the roadway for another vehicle, a traffic signal adjacent the entrance ramp signals for a vehicle to enter the highway. This normal system operation is modified to accommodate extremely congested or extremely light traffic conditions.

BACKGROUND OF THE INVENTION This invention pertains to a highway traflic control system. More particularly, this invention pertains to a system for controlling the entrance of vehicles onto a limited access highway from an entrance ramp leading to the limited access highway from an adjacent local street or frontage road.

Heavy traffic demands are placed upon the street systems of large metropolitan areas, particularly during the periods in the morning and afternoon when large numbers of commuters are traveling to and from work, and during other special times. This traffic frequently travels substantial distances from the outskirts of a large city to the downtown central business district, and, to enable it to travel more rapidly, numerous cities have developed systems of limited access roadways frequently referred to as expressways, freeways, or parkways. These expressways are high cost facilities designed for heavy traffic flow at relatively high speeds. During the peak trafiic periods when the commuter traffic is heaviest, even these expressways become congested due to the large amount of traffic using them, breaking down the free flow of traffic.

Limited access roadways generally pass under or over the city streets which make up the remainder of the arterial street system. Trafiic seeking to enter an expressway is routed onto it via an entrance ramp from an adjacent frontage road which may be an arterial street of the system. The expressway trafiic is generally moving at a relatively high speed, for example in the range of 50 to 60 m.p.h. When the expressway traffic is heavy, a vehicle attempting to enter it from an entrance ramp frequently must stop to wait until there is sufficient available capacity on the expressway to accommodate the entering vehicle. To be able to adequately observe the expressway traffic, the driver of the entering vehicle usually finds it necessary to wait near the end of the entrance ramp, adjacent the merging area of the entrance ramp and the expressway. Since the merging vehicle must start from a standstill, it has to accelerate rapidly to the speed of the expressway traffic. Consequently, a substantial opening or break in the expressway trafiic is necessary to enable the entering vehicle to reach the required speed and to merge safely. This means that a large amount of available capacity must exist for the entering vehicle to safely ice merge onto the expressway. Thus, available expressway capacity is not fully utilized.

When the expressway traffic is heavy, it is frequently necessary for an entering vehicle on the entrance ramp to wait for a long period of time before there is sufficient available capacity on the expressway to accommodate it. As a consequence, a line of vehicles frequently accumulates on the entrance ramp, and this line often grows to such a length that it stretches into the adjacent frontage road and adds to the congestion on it and on the remainder of the arterial street system. As a consequence, congestion on the expressway and on the arterial streets is increased, and the drivers seeking to travel faster by means of the expressway must spend considerable time waiting to enter it. Therefore, the desired result of the expressway, namely to reduce overall travel time, is not achieved.

Attempts have been made to reduce the traffic congestion at the expressway entrance ramps by closing selected entrance ramps during certain periods of peak traffic congestion. While this somewhat improves the traffic flow on the expressway itself, and it avoids lines of waiting vehicles on the local streets adjacent these expressway entrance ramps, it diverts vehicles which otherwise would utilize the expressway onto the arterial streets of the local street system. As a consequence, these ve hicles require greater travel time because they are unable to utilize the expressway, and traffic on the local arterial streets is increased with a resulting increase in congestion there.

Other attempts to improve traffic flow at expressway entrance ramps have included the use of trafiic control personnel to determine when there is sufficient unused capacity on the expressway to accommodate a vehicle from the entrance ramp and to then signal to a waiting driver on the entrance ramp to enter the expressway. This simply replaces the judgment of the driver with the judgment of the traffic control personnel. Errors in judgment by the controlling personnel or failure of a waiting driver to heed the signals of the control personnel can result either in available capacity on the expressway being unused because no driver enters it or in a collision of vehicles in the merging area of the entrance ramp and the expressway because a driver attempts to merge onto the expressway when there is not sufficient available capacity.

Another control system which has been developed f r an expressway entrance ramp simply provides an indication to vehicles on the entrance ramp which alternates between green or proceed and red or stop at a fixed cyclic rate. Thus, in every sixty seconds, the traffic on the entrance ramp is assured of receiving 2. proceed indication for a fixed number of seconds, whether there is sufficient available capacity on the expressway to accommodate the vehicles which can enter during this time or not. Such a system does not assure that vehicles which have been signalled to proceed toward the merging area of the entrance ramp and the expressway fiind sufficient capacity on the expressway to permit them to merge when they arrive there. Modifying the time ratio of the proceed and stop indications in accordance with the speed, volume or density of traffic on the expressway merely reduces the amount of congestion at the merging area; it does not assure that entering vehicles will be able to merge.

A system for control of expressway entrance ramps has been proposed in which the volume of traffic already on the expressway is subtracted from an empirical value representing the maximum volume of traffic which the expressway can accommodate. This difference between expressway capacity and expressway demand is integrated with time, and when this time integral indicates that there exists sufiicient available capacity on the expressway to accommodate another vehicle, a traffic signal adjacent the entrance ramp signals for a vehicle to enter, and the integrator is reset. This proposed system assures that the expressway capacity is fixed. However, when the expressway is congested it can accommodate fewer vehicles than when it is not congested. Hence, the expressway capacity is lower when traffic congestion exists than when traffic is flowing smoothly. Thus, at lower expressway speeds, the maximum traffic volume which the expressway can accommodatei.e., the expressway capacity is lower. This expressway congestion may be due simply to the large volume of traffic wanting to use the expressway, or it may be due to some unusual condition such as a trafiic accident on an icy road surface. In any event, the expressway congestion is reflected in a lower average expressway traffic speed and in a lower expressway traffic volume, and the congestion results in a lower expressway capacity.

SUMMARY OF THE INVENTION In one aspect, the present invention is a system for controlling the release of vehicles onto a limited access roadway from an entrance ramp which terminates in a merging area with the roadway. The system is adapted for use with vehicle sensing means of the type which provides an indication of vehicle presence and speed and for use with a traffic signaling means which controls release on the entrance ramp of vehicles heading toward the mergging area. The tralfic signaling means can be located adjacent to the entrance ramp a distance from the merging area, and so entering vehicles can accelerate some distance before reaching the merging area. This allows a vehicle to merge into a smaller opening in the expressway traffic than is possible when a vehicle commences acceleration in the merging area. As a consequence, a smaller amount of available capacity on the expressway is required for the entering vehicle. The system includes means to detect the volume of traffic already using the expressway, means to determine the amount of traffic volume which the expressway can accommodate at each instant of time in accordance with conditions on the expressway, and means to signal to a waiting vehicle on the entrance ramp when there is sufiicient available capacity on the expressway to accommodate that waiting vehicle.

Another aspect of the present invention includes means to continuously monitor the available capacity on the expressway after it has reached a level at which another vehicle from the entrance ramp could enter the expressway to insure that changed traffic conditions do not result in a decrease in the available capacity below the required amount before a vehicle attempts to enter the expressway from that entrance ramp.

A further aspect of the invention includes means to determine the amount by which the maximum roadway capacity is reduced due to the instantaneous roadway conditions, means to measure the volume of traffic on the roadway upstream of the entrance ramp to provide a diiference between instantaneous roadway capacity and instantaneous roadway demand which difference may be either positive or negative indicating available capacity or shortage of capacity respectively, means to integrate this volume difference, means to indicate a permissive condition when the integrated volume difference indicates the presence on the roadway of suflicient available capacity for a vehicle from the ramp to merge onto the roadway, whereby the permissive condition resets the integrator and enables the signaling of a proceed indication on the entrance ramp, provided a vehicle has been detected on the ramp waiting to merge onto the roadway, and means to maintain the permissive condition for a maximum time in the event no vehicle is present on the ramp at the time the permissive condition is initiated,.

while the integrator again monitors the accumulated available capacity on the roadway.

Another aspect of the present invention includes means to override the operation of the control system so that during periods of extremely great congestion on the expressway, no vehicles are permitted to enter from the entrance ramp, while during periods of extremely light traffic on the expressway, vehicles are at all times permitted to enter the expressway from the entrance ramp without any metering.

It is accordingly an object of the present invention to provide an improved system for controlling the merger of traffic onto a limited-access roadway from an entrance ramp.

It is another object of the present invention to provide a system for controlling entrance of vehicles onto an expressway from an entrance ramp in response to the existence of sufficient available capacity on the expressway, considering instantaneous conditions on the expressway.

It is a further object of the present invention to provide an improved system for determining the availability on an expressway of sufficient unused capacity to accommodate an additional vehicle, in view of the traflic volume already on the expressway and of the instantaneous conditions on the expressway, and for signaling for a waiting vehicle to merge onto the expressway from an entrance ramp when such sufficient unused capacity is measured.

It is an additional object of the present invention to provide a system for measuring one or more traffic characteristics, such as traffic speed, traflic volume, traflic density for example, which affects the capacity of an expressway and for modifying the indicated capacity of that expressway in accordance with those characteristics and for signaling for an additional vehicle to enter the expressway when the traffic volume already on the expressway is sufliciently below the net expressway capacity to permit merger of that additional vehicle.

It is yet another object of the present invention to provide a system for storing for a maximum time a permissive-condition indication in response to presence on an expressway of sufficient available capacity to accommodate an additional vehicle, and for providing a proceed indication for a vehicle seeking to enter the expressway from an entrance ramp during the time that permissivecondition is stored.

It is still a further object of the present invention to provide such a control system for a limited-access roadway entrance ramp which includes provision to override regular operation of the control system during times of very light traflic and during times of extreme traffic congestion to insure optimum performance of the roadway and its contiguous surface road network.

These and other objects and advantages will be appar ent in the present invention from the following detailed description and claims, particularly when read in conjunction with the accompanying drawings in which like parts bear like reference designations.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a diagram depicting a limited access roadway having an entrance ramp which leads from a frontage road to the limited access roadway and which is suitable for operation under control of the present invention.

FIGS. 2A and 2B form a block diagram of a preferred embodiment of the present invention when they are joined together as shown in FIG. 5.

FIG. 3 is a schematic diagram of a timer and a switching unit suitable for use in the embodiment of FIG. 2.

FIG. 4 is a block diagram depicting a signal controller suitable for use in the embodiment of FIG. 2, and showing the integrator, including its reset circuit, of that embodiment.

FIG. 5 shows how FIGS. 2A and 2B are joined to depict a preferred embodiment of the invention.

DESCRIPTION OF A PREFERRED EMBODIMENT FIG. 1 depicts a roadway 10 forming one side of a limited access divided roadway, the other side being on the far side of median strip 12 which is beyond the edge 14 of roadway 10. Traffic on roadway moves to the right as shown by the arrow, where the drive to the right rule prevails, while traffic in the opposite direction travels on the other side (not shown) of the dual roadway. Each side of the divided roadway has several lanes for travel, and by way of example in FIG. 1 roadway 10 is shown as having three lanes of travel.

Entrance ramp 16 connects the adjacent frontage road 18 to roadway 10, and cars traveling on frontage road 18 are able to enter roadway 10 by means of the ramp as indicated by the arrow on ramp 16. By way of example, frontage road 18 might be a part of the arterial street system in a large urban area, while limited access roadway 10 might be part of an expressway, freeway, or parkway through or approaching the urban area. While frontage road 18 is shown parallel to roadway 10 in FIG. 1, it might run at an angle to roadway 10, passing over or under it by means of a bridge or underpass.

Merging area 20 is provided at the end of entrance ramp 16, where it joins roadway 10. This merging area 20 enables cars entering roadway 10 from the entrance ramp 16 to travel for a short time parallel to roadway 10 before actually entering its right hand lane. As depicted in FIG. 1, frontage road 18 is provided for one way travel to the right, as shown by the arrow, and a second lane 22 is provided on frontage road 18 immediately before the entrance ramp 16 for the convenience of vehicles turning onto entrance ramp 16 from the frontage road 18, but such a turning lane might not be provided and frontage road 18 could be a two way street.

A traffic signal 24 is mounted adjacent entrance ramp 16, at a point between frontage road 18 and merging area 20, for example about half way between the frontage road and the merging area. Traific signal 24 might be a standard red-yellow-green traffic light, for example. Alternatively, it might be any other type of traffic control signal or device, such as a gate that rises and falls across entrance ramp 16.

A number of vehicle sensing means or vehicle detectors monitor the trafiic fioW on roadway 10, entrance ramp 16, and merging area 20. As depicted in FIG. 1, vehicle detectors D1, D2 and D3 are located at a distance upstream from merging area 20 to monitor the trafiic in the right hand lane, the center lane, and the left hand lane of roadway 10, respectively, as that traffic approaches merging area 20.

The detection zones of vehicle detectors D1, D2, and D3 are depicted in FIG. 1 by the outlines D1, D2 and D3 on the right hand lane, the center lane, and the left hand lane of roadway 10, respectively, and as shown there each detection zone covers a brief length of the corresponding lane of roadway 10.

Each of the vehicle detectors D1D3 indicates the passage of vehicles through its detection zone, and each detector Dl-D3 indicates the speed of each vehicle which passes through its detection zone. Detectors D1-D3 may each comprise two separate units to perform these two functions, or the two functions may be performed by a single unit for each detector D1-D3.

Each detector D1D3 might be any one of several types which are capable of providing the required information about passing vehicles. For example, each may be a sonic detector, such as that disclosed and claimed in co-pending applicationu Ser. No. 551,692 by Bernard J. Midlock, entitled Sonic Vehicle Detector filed May 20, 1966 and assigned to the same assignee as the present invention. Alternatively, each detector Dl-D3 could be an inductive loop detector such as a wire loop and an LD-l Loop Vehicle Detector manufactured by Automatic Signal Division of Laboratory Electronics, Inc. and described in that companys LD-1 Loop Vehicle Detector Bulletin D--168 copyright 1966 by Automobile Signal Division of Laboratory for Electronics, Inc. Other equivalent vehicle detectors might be used instead of the above representative examples.

Vehicle detectors D4, D5 and D6 are mounted over or in the right hand lane, the center lane, and the left hand lane, respectively, of roadway 10 a distance downstream from entrance ramp 16 so that these detectors D4D6 monitor vehicles on roadway 10 after they have passed the merging area 20. Each of these detectors D4-D6 indicates the passage of vehicles and the speed of vehicles, just as do the detectors D1-D3, and by way of example each of the vehicle detectors D4-D6 might be either the above-described sonic vehicle detector or the above describer loop vehicle detector. The detection zones of the vehicle detectors D4, D5 and D6 are depicted in FIG. 1 by the outlines D4, D5 and D6, respectively, and as shown there each detection zone covers a brief length of its corresponding lane of roadway 10.

Vehicle detector D7 is mounted in a position to monitor vehicles in the right hand lane of roadway 10 just upstream of merging area 20. Vehicle detector D7 is required to give an output indicative of the speed of vehicles passing through its detection zone which is depicted in FIG. 1 by the outline D7. By way of example, vehicle detector D7 could be either a sonic vehicle detector or a loop vehicle detector, as described above.

Detector D8 is mounted in a position to monitor vehicles as they pass through merging area 20. When a vehicles passes through the detection zone of vehicle detector D8 (indicated by outline D8, FIG. 1) the detector provides an output indicative of the speed of that vehicle, and when a vehicle stops in the detection zone of detector D8, the detector provides a continuous or long term output indicative of the presence of that vehicle. Detector D8 could be any one of several types capable of providing the required signals. By way of example, it could be either the sonic detector or the inductive loop detector described above with reference to detectors D1-D3. To insure coverage of the entire merging area 20, two or three detectors might be used, connected in parallel.

Vehicle Detector D9 is mounted in a position to monitor vehicles on entrance ramp 16 immediately in front of tratfic signal 24. Detector D9 is required to provide an indication of the presence of vehicles within its detection zone (shown in FIG. 1, outline D9), but it need not provide vehicles speed information. By way of example, detector D9 could be either a sonic detector or an inductive loop detector as described above with reference to detectors D1D3.

When FIGS. 2A and 2B are placed side by side as shown in FIG. 5, so that the like-numbered lines are joined, they comprise FIG. 2 which depicts a preferred embodiment of the present invention. Detectors D1-D9 and traffic signal 24 are connected to traffic control circuitry as shown in FIG. 2. This trafiic control circuitry might be located in a control box adjacent to the entrance ramp 16, or alternatively it might be located remote from entrance ramp 16, by way of examples.

As depicted in FIG. 2, vehicle detectors D2, D5, D7, and D8 are connected to speed computers 30, 32, 34, and 36 respectively. Each of these detectors provides to its speed computer signals which are indicative of the speed of Vehicles passing through the detection zone of the detectors. Each of these speed computers develops an output which is indicative of the average speed of vehicles passing its detector. Suitable speed computers are known in the art. By way of example, one speed computer suitable for use as speed computers 30, 32, 34, and 36 is disclosed and claimed in the co-pending application Ser. No. 617,555 entitled Speed Averaging Circuit by Ludwig Pallat filed Feb. 21, 1967 and assigned to the same assignee as the present invention. The speed computer there disclosed and claimed requires for each passing vehicle an input pulse having a duration which is inversely proportional to the speed of that passing vehicle. The speed computer provides an output voltage level proportional to the average speed of passing vehicles. The sonic vehicle detector and the inductive loop vehicle detector described above with reference to vehicle detectors DlD3 provide the necessary input signals for such a speed computer. As another example, US. Pat. 3,059,232 issued Oct. 16, 1962 to I. L. Barker and assigned to the same assignee as the present invention discloses a speed averaging circuit suitable for use as speed computers 3tl36 when the associ ated vehicle detectors are Doppler radar vehicle detectors of the type disclosed in that same patent.

Speed computer 34, which is associated with vehicle detector D7, has its output connected to diiferentiator 35, which by way of example might comprise a standard resister-capacitor differentiating circuit. The output of differentiator 35 is coupled through inverter 37 to weighing unit 39. By way of example the output of inverter 37 could be applied to one side of the fixed resistance of a potentiometer, the other side of which is tied to ground. The potentiometer then constitutes weighing unit 39, with its output taken from the potentiometer arm. Inverter 37 is depicted by way of example in FIG. 2 as comprising an operational amplifier with an appropriate input resistance and an appropriate feedback resistance.

The output of each of the speed computers 3036 is connected to a level monitor circuit capable of providing discrete outputs indicative of the level of the applied input signal. Thus, by way of example, level monitor 38 which is connected to speed computer 30 has three output lines, depicted in FIG. 2 by line 46. The first of these output lines is connected to one input of AND gate 48. The second output line is connected via line 47 to timer 50, while the third output line from level monitor 38 is connected to one input of AND gate 52. Thus, if the speed of vehicles passing vehicle detector D2 is within a first range, as indicated by the voltage level applied from speed computer 30 to level monitor 38, then level monitor 38 applies a signal on its first output line to AND gate 48. If the speed applied from speed computer 30 to level monitor 33 is Within a second range, then level monitor 38 applies a signal on its second output line to the input of timer 50. If the speed indicated by speed computer 30 is within a third range, then level monitor 38 applies a signal on its third output line to the input of AND gate 52.

Similarly, level monitor 40 has a plurality of output lines, represented in FIG. 2 by line 54. Level monitor 40 divides the speed information provided to it from speed computer 32 into discrete levels, and the level monitor energizes one of its output lines 54 accordingly. Thus, when the speed of vehicles passing vehicle detector D is within a first range, as indicated by the signal applied from speed computer 32 to level monitor 40, then level monitor 40 applies a signal on its output line 55 to selector 56. If the speed signal applied by speed computer 32 to level monitor 46 is within a second range, then level monitor 40 applies a signal on another of its output lines to timer 58, and if the speed signal applied by speed computer 32 to level monitor 40 is within another range, then level monitor 40 applies its output signal on its other output line to the second input of AND gate 52.

In like manner, level monitor 42. divides the speed indications from speed computer 34, indicative of the average speed of vehicles passing vehicle detector D7, into discrete speed level indications. Thus, when the speed of vehicles passing vehicle detector D7, as indicated by the output of speed computer 34, is within a first range, then level monitor 42 applies an output on its output line 60. When the output of speed computer 34 indicates that the speed of vehicles passing detector D7 is within another range, then level monitor 42 applies an output on line 62. When the speed indication from speed computer 34 to level monitor 42 is within a third range, then level monitor 42 applies an output to the second input of AND gate 48.

Level monitor 44 divides the speed indications received from speed computer 36, indicative of the speed at which vehicles are passing vehicle detector D8 within merging area 20, into a plurality of discrete output levels, for example eight speed levels. Each of these outputs is applied by a separate output line from level monitor 44 to weighting unit 64. The plurality of output lines from level monitor 44 to weighting unit 64 are depicted in FIG. 2. as line 66. By way of example, weighting unit 64 might comprise a plurality of otentiometers, each having its fixed resistance connected between a source of positive potential and a source of negative potential, relative to the system ground at terminal 117. Each potentiometer armature is tied to its own normally open relay contact. All of the associated moving relay contacts are tied together, and to output line 65 from weighting unit 64. Each relay coil is connected to an associated one of the input lines 66. Thus, the potential on line 65 is dependent upon the setting of the potentiometer associated with the relay energized under control of the output line 66 from level monitor 44 corresponding with the level of the average speed signal from speed computer 36. Obviously, solid state switching might be utilized in place of relays, or other designs of a weighting unit could be utilized.

The ranges into which level monitors 3844 divide their respective input singals may overlap, or they may be mutually exclusive, depending upon the adjustment of the respective level monitors. The level ranges need not be contiguous, but may be separated by level ranges in which no level monitor output is provided. The particular adjustment of each level monitor depends upon the requirements of the particular entrance ramp under control.

Numerous circuits are known which can be utilized as level monitors 3844. One circuit suitable for this use is disclosed in US. Pat. No. 2,932,003 issued Apr. 5, 1960 to John L. Barker and assigned to the same assignee as the present invention, provided that voltage source 469 shown in FIG. 4 of that patent is either a negative voltage source or ground, with suitable connections made from the second side of each indicator 480-485 in FIG. 4 of that patent.

AND gate 48 receives an input when the speed of the vehicles passing the vehicle detector D2 is at a low level, as determined by level monitor 38. AND gate 48 also receives an input when the speed of vehicles passing vehicle detector D7 is at a low level, as determined by level monitor 42. Coincidence of these two inputs to gate 48 results in a signal being applied to the input of timer 6%. Timer 68 times a minimum duration and then turns on switching unit 70, applying a positive potential on output line 71 from unit 70.

AND gate 52 receives an input from level monitor 38, indicating that the speed of vehicles passing vehicle detector D2 is high. Gate 52 receives a second input from level monitor 40, indicating that the speed of vehicles passing vehicle detector D5 is low. Coincidence of these two inputs to gate 52 results in a signal being applied to the input of timer 72. Timer 72 times a minimum duration for this condition and then turns on switching unit 74, applying a positive potential on its output line 75.

Vehicle detector D3 is connected to the input of timer 76 so that so long as a vehicle is within the detection zone of vehicle detector D8 a signal is applied to the input of timer 76. Timer 76 times a minimum duration for this condition and then turns on switching unit 78, applying a positive potential on its output line 79.

Timer 50 receives an input from level monitor 38 when the average speed of vehicles passing vehicle detector D2 is within the low speed range determined by the adjustment of level monitor 38. If this input continues for the length of time required for timer S0 to time out, the timer applies a signal to switching unit 80, causing the switching unit to apply a positive potential on its output line 81.

Timer 58 receives an input from level monitor 40 when the average speed of vehicles passing vehicle detector D is within the low speed range determined by the adjust ment of level monitor 40. If this input continues for the length of time required for timer 58 to time out, the timer applies a signal to switching unit 82, causing the switching unit to apply a positive potential on its output line 83.

The outputs of the vehicle detectors D1, D2, D3, D4, D5 and D6 are connected to the inputs of volume computers 84, 86, 88, 90, 92 and 94, respectively. Each of these volume computers generates an output indicative of the volume of traffic flow past its associated vehicle detector. Numerous such volume computers are known in the art. By way of example, one volume computer suitable for this application is disclosed in US. Pat. 2,932,003 issued Apr. 5, 1960 to J. L. Barker and assigned to the same assignee as the present invention. The output suitable for the present application is found on line 345 in FIG. 3 of that patent. The volume computer there disclosed requires that with each vehicle passing the vehicle detector, the input line to the volume computer be grounded. This requirement is met by the sonic vehicle detector and by the inductive loop vehicle detector described above with reference to vehicle detectors D1-D3, as well as by numerous other vehicle detectors.

The outputs of volume computers 84, 86, and 88, associated with vehicle detectors D1, D2, and D3 respectively, are applied as inputs to summing amplifier 96, which is depicted by way of example in FIG. 2 as comprising an operational amplifier with appropirate input and feedback resistors. Similarly, the outputs of volume computers 80, 92 and 94, associated with vehicle detectors D4, D5 and D6 respectively, are applied as inputs to summing amplifier 98, which by way of example is depicted in FIG. 2 as comprising an operational amplifier with appropriate input and feedback resistors. Because of the invention which is inherent in the operation of the summing amplifiers, the output of summing amplifier 96 is a voltage representing the negative of the total traffic volume demand on roadway passing the upstream vehicle detectors Dl-D3 Similarly, the output of summing amplifier 98 is a voltage representing the negative of the total downstream trafiic volume passing vehicle detectors D4D6. This output from summing amplifier 98 is applied through inverting amplifier 100, which is depicted by way of example in FIG. 2 as comprising an operational amplifier with an appropriate input and an appropriate feedback resistor. Consequently, the output of amplifier 100 is a voltage representing the total downstream traffic volume.

The output of summing amplifier 96 and the output of inverting amplifier 100 are applied as inputs to summing amplifier 102, which is depicted in FIG. 2 by way of example as comprising an operational amplifier with appropriate input and feedback resistors. Again, an inversion occurs in summing amplifier 102, and so its output is a voltage representing the diiference between the upstream trafiic volume and the downstream traffic volume. This output from summing amplifier 102 is applied as an input to level monitor 104, wthich can be of the same type as described above with reference to level monitors 38-44.

When the difference between the upstream traffic volume and the downstream traific volume is within a first range, level monitor 104 applies an output to timer 106 which times a minimum duration of this condition and then turns on switching unit 108 to apply a positive potential to output line 109 from unit 108. When the volume ditference is within a second range, level monitor 104 applies an output on its second output line 110. These two ranges associated with level monitor 104 do not overlap and need not be contiguous.

FIG. 3 is schematic representation of a timer and switching unit which can be used in the embodiment of FIG. 2. The circuit of FIG. 3 is suitable for use as each timer and switching unit combination, i.e., timer 68 and unit 70, timer 50 and unit 80, timer 72 and unit 74, timer 58 and unit 82, timer 76 and unit 78, and timer 106 and unit 108. The input to the timer is applied by line 151 to the junction of serially connected resistors 152 and 154. Line 151 represents the input line to each timer, such as line 47 connecting level monitor 38 to timer 50 in FIG. 2, for example. Resistor 152 has its other end tied to a source of positive potential, such as +15 volts D.C. Resistor 154 has its other end tied to the first end of resistor 156, the second end of which is connected to a source of negative potential, such as -15 volts DLC. The junction of resistors 154 and 156 is tied to the base of NPN transistor 158, which has its collector connected through resistor 160 to the souce of positive potential. The emitter of transistor 158 is coupled through resistor 162 to the source of negative potential. The emitter of transistor 158 is also coupled to the source of negative potential through serially connected resistors 164 and 166 which have their junction tied to the base of NPN transistor 168. The collector of transistor 168 is connected to one end of variable resistor 170 which has its other end tied to the arm of potentiometer 172. The fixed resistance of potentiometer 172 is connected between the source of positive potential and ground. The emitter of transistor 168 is tied to ground.

The collector of transistor 168 is also connected to the first plate of capacitor 174 which has its second plate grounded. The first plate of capacitor 174 is also applied to the base of NPN transistor 176. The collector of transistor 176 is coupled to the source of positive potential through resistor 178. The emitter of transistor 176 is connected to ground through resistor 180. The collector of transistor 176 is connected to the base of NPN transistor 182 which has its emitter tied to the emitter of transistor 176. The base of transistor 182 is connector to ground through resistor 184. The collector of transistor 182 is connected to the source of positive potential through serially connected resistors 186 and 188 which have their junction tied to the base of PNP transistor 190 within the switching unit. Transistor 190 has its emitter tied to the source of positive potential and its collector coupled to the source of negative potential through serially connected resistors 192 and 194. The junction of resistors 192 and 104 is tied to the base of NPN transistor 196 which has its emitter tied to ground. Current limiting resistor 198 and the fixed resistance of potentiometer 200 are connected in series between the positive voltage source and ground, and their junction is tied to the collector of transistor 196. The arm of potentiometer 200 is tied to the output line 2010f the switching unit. Line 201 represents the output line of each switching unit, such as output line 81 from switching unit 80 in FIG. 2, for example.

With no input applied to the timing circuit, transistor 158 conducts. This causes transistor 168 to conduct, and so its collector is at substantially ground potential. As a consequence, no voltage can build up on capacitor 174. Thus, transistor 176 is cut off, and transistor 182 conducts. Current flowing through transistor 182 holds transistor 190 on, and so a positive potential is applied to the base of transistor 196 holding it on. As a consequence, both sides of the fixed resistance of potentiometer 200 are at approximately ground potential, and the circuit output is ground potential.

The input signal applied to the timer circuit constitutes the application of ground or of a negative potential to the input line 151. When this occurs, the voltage on the base of transistor 158 drops, causing its emitter voltage to drop. As a consequence tranistor 168 cuts off. Voltage then builds up on capacitor 174 at a rate determined by the size of capacitor 174 and by the settings of variable resistor 170 and potentiometer 172. When this voltage has built up to a high enough level, the Schmitt trigger comprising transistors 176 and 182 fires. This turns off transistor 182,

and so transistor 190 is cut off. As a result the potential on the base of transistor 1'96 drops, and that transistor turns otf, causing the voltage on the collector of transistor 196 to rise from substantially ground level to substantially full positive supply level. Therefore, the circuit output line 201 rises to a positive voltage determined by the setting of potentiometer 200.

Turning again to FIG. 2, scaling unit 116 has its input line 115 applied to a suitable source of potential, which for example might be 15 volts DC. from terminal 114 relative to ground at terminal 117. Scaling unit 116 might comprise a potentiometer, by way of example, thus enabling the output of the scaling unit to be set to any voltage level between 15 volts DC. and ground.

The outputs of the switching units, 70, 74, 80, 82 and 108 and the outputs of the weighting units 64 and 39 are connected to one contact of single-pole, single- throw switches 118, 120, 122, 124, 126, 128, and 130, respectively. The second contact of each of these switches 118130 is connected as an input to summing amplifier 132. The output of scaling unit 116 is also connected as an input to summing amplifier 132. Thus, the output of summing amplifier 132 is the algebraic sum of the outputs of switching units 70, 74, 80, 82 and 108, the outputs of weighting units 3 9 and 64, and the output of scaling unit 116. Summing amplifier 132 might for example comprise an operational amplifier with appropriate input and feedback resistors, as depicted in FIG. 2.

The output of scaling unit 116 is a voltage representing an empirical value of capacity for roadway 10. Where, as in FIG. 2, this capacity is compared with the trafiic volume demand on the roadway, as represented by the outputs of volume computers 84-88, the output of scaling unit 116 represents the trafiic volume capacity of roadway 10, but other measures of roadway capacity and demand could be utilized in place of traffic volume. The output of each of the switching units 70, '74, 80, 82, and 108 and of each of the weighting units 39 and 64 is a voltage representing a parameter indicative of congestion on roadway 10 which results in variation in this empirical value of the capacity of roadway 10. These variations in the roadway capacity reflect changing traffic conditions on roadway 10.

Congestion is used herein in a relative sense rather than in as absolute sense. Thus, congestion is not limited to a complete blockage of the roadway or to extremely low trafiic speeds-stop and go conditions. As used herein congestion includes varying degrees of medium to high density, medium to low speed traflic conditions, for example, reflecting restraints on roadway capacity. Such conditions might be found for example during peak traffic periods such as morning and evening rush hours when commuter traflic is heavy, in addition to being found as as result of a blockage of one or more of the lanes of the roadway due to a collision or other unusual ccurrence downstream of the entrance ramp. Further, congestion as used herein includes conditions on the roadway approaching an area of restriction on free traffic flow.

The first parameter that provides an indication of congestion on roadway 10 liable to affect the entrance of vehicles from ramp 16 is low average speed downstream of ramp 16. Thus, low speed of vehicles passing vehicle detector D causes level monitor 40 to apply an input to timer 58 which turns on switching unit 82 after it has timed out. Switching unit 82 then applies a voltage through switch 124 in its closed position to reflect this reduction in the capacity of roadway 10.

Continued build up of congestion downstream of ramp 16 results in a decrease in the traffic volume passing the downstream vehicle detectors D4-D6, relative to the traffic volume passing the upstream detectors D1-D3. The output of summing amplifier 102 represents the difference between the total upstream traflic volume and the total downstream traflic volume. Level monitor 104 turns on timer 106 when this volume difference indicates that traffic congestion is commencing to build up downstream of the entrance ramp. If this indicated congestion buildup continues for the length of time for which timer 106 is set, then timer 106 causes switching unit 108 to apply a voltage through switch 126 in its closed position to summing amplifier 132 to reflect this indication of reduced roadway capacity.

If the downstream congestion continues to build up, the average speed of traflic passing vehicle detector D7, just upstream of merging area 20, drops. The speed of vehicles passing vehicle detector D7 is determined by speed computer 34. This speed indication is then applied to diiferentiator 35. If the average speed of traffic passing detector D7 is dropping, then the output from differeniator 35 is a negative voltage. This output is passed through inverting amplifier 37 to weighting unit 39. The output from weighting unit 39, which reflects this change in the average speed of vehicles nearing the merging area, is connected through switch 130 in its closed position to the input of summing amplifier 132 to reflect a decrease in the roadway capacity.

The speed with which vehicles are passing through merging area 20 reflects the ease with which these entering vehicles are able to merge into the trafiic on roadway 10. If this speed is relatively high, then the entering vehicles are merging easily, and it would be possible for them to merge in a satisfactory manner with less available capacity. However, if the speed of these entering vehicles is low, then it indicates that it is difficult for them to merge, and so a greater available capacity should be indicated before they are permitted to merge. Vehicle detector D8 which determines the speed of vehicles passing through merging area 20 is connected to the input of speed computer 36. The output of speed computer 36 is tied to level monitor 44 which divides this vehicle speed information into a plurality of level ranges, for example eight levels. These eight outputs from level monitor 44 are connected via lines 66 to weighting unit 64. The output of weighting unit 64 is a positive or a negative voltage, having its magnitude and polarity determined by the output from level monitor 44 under the control of speed computer 36. Thus, if the output of speed computer 36 indicates that vehicles from ramp 16 are rapidly passing through merging area 20, weighting unit 64- applies a negative voltage through switch 128 in its closed position to summing amplifier 132 to increase the indicated capacity of roadway 10. However, if the computer 36 output indicates that vehicles are having difliculty merging onto roadway 10, then unit 64 applies a positive voltage to amplifier 132 to reduce the indicated roadway capacity.

The average speed of trafl'ic upstream of ramp 16 does not drop as soon as the downstream average trafiic speed If the speed of vehicles passing vehicle detector D2 is high while the speed of vehicles passing vehicle detector D5 is low, congestion has commenced downstream of the entrance ramp but has not yet reached the upstream detectors D1-D3. In this situation, level monitors 38 and 40 apply inputs to AND gate 52 which turns on timer 72. This timer times a minimum duration for this indication of traflic congestion and then causes switching unit 74 to apply a voltage through switch 120 in its closed position to the input of summing amplifier 132 to reflect the reduced roadway capacity.

If the trafiic congestion reaches the upstream vehicle detectors Dl-D3, the average trafiic speed there drops. When the speed of vehicles passing vehicle detector D2 drops to a low level, level monitor 38 applies a signal to timer 50. If this low speed level continues for the length of time required for timer 50 to apply a signal to switching unit 80, then switching unit connects a voltage through switch 122 in its closed position to the input of summing amplifier 132. This reduces the indication of the capacity of the roadway which is represented by the output of summing amplifier 132.

Under this condition of high trafiic congestion, speed along roadway is low. When the speed of vehicles passing vehicle detector D2 is low and the speed of vehicles passing vehicle detector D7 is low, AND gate 48 turns on timer 68 which times a minimum duration for this indication of congestion and then causes switching unit 70 to apply a voltage through switch 118 in its closed position to summing amplifier 132 to reflect a further reduction in capacity of roadway 10.

The output of summing amplifier 132 represents the instantaneous traffic volume capacity of roadway 10 as determined by modifying the empirical capacity set at scaling unit 116 by all of the roadway congestion indicative parameters received through switches 118-130. This instantaneous roadway capacity from summing amplifier 132 is applied through selector 56 to the first input of summing integrator 134. The output of summing amplifier 96 is a voltage representing the negative of the traific volume demand on roadway 10 passing the upstream vehicle detectors D1D3. This output is applied via line 97 to the other input of summing integrator 134. Summing integrator 134 is depicted by way of example in FIG. 2 as comprising an operational amplifier with appropriate input resistors 133 and 133 and appropriate feedback capacitor 135. The output of the summing integrator 134 is the time integral of the difference between the instantaneous roadway traffic volume capacity and the upstream trafiic volume demand. Thus, this output from integrator 134 reflects the accumulated available unused capacity on roadway 10. The available capacity indication from integrator 134 is applied via line 137 as an input to signal controller 136 which is connected to traffic signal 24 by suitable lines, depicted in FIG. 2 by line 139. When the time integral indicates that sufiicient available capacity exists on roadway 10 for a vehicle from entrance ramp 16 to merge onto roadway 10, controller 136 applies suitable signals via lines 139 to cause traffic signal 24 to indicate to a driver waiting on the entrance ramp that he should proceed toward merging area 20. At that time signal controller 136 applies a signal via lane 141 to reset circuit 138 to cause it to reset integrator 134 so that the integrator can again commence measurement of available traffic capacity.

FIG. 4 depicts one embodiment of a signal controller which is suitable for use as signal controller 136. As shown in FIG. 4, the output of integrator 134 is applied via line 137 to the signal input of INHIBITED-AND gate 202. Gate 202 has its output tied to the input of Schmitt trigger 204 which feeds monosta-ble multivibrator or one shot 206. The output of one shot 206 is connected to the set input of bistable multivibrator or fiipflop 208. The set output of fii-pflop 208 is connected to the inhibit input of INHIBITED-AND gate 202. Thus, if fiipfiop 208 is in its reset condition the output of integrator 134 passes through gate 202 to the input of Schmitt trigger 204. When the integrator output reaches the firing level of Schmitt trigger 204, the trigger fires one shot multivibrator 206 to set flipflop 208. The set output of fiipflop 208 then pevents any further signals from integrator 134 from passing through gate 202.

The set output of flipflop 208 is applied via line 141 to reset circuit 138 in which it triggers one shot multivibrator 210 which turns on relay driver 212 to energize relay 214. Normally open contact 216 of relay 214 then closes momentarily to short the integrating capacitor 135 of integrator 134. The relay then drops out, and contact 216 opens so that integrator 134 can again build up voltage. This voltage build-up is prevented from passing through gate 202 so long as fiipflop 208 is in its set condition. Of course, one shot 210 and relay driver 212 might be combined into one unit. Alternatively, a solid state switching device might be utilized to reset integrator 134 when flipflop 208 assumes its set condition.

Flipfiops 218, 220, and 222 control the excitation of the red indication, the yellow indication, and the green indication, respectively, on trafiic signal 24. The set output of flipflop 218 passes through INHIBITED-AND gate 224 to line 139R to cause the red indication. Line 139R also applies this signal to a signal input of INHIBITED- AND gate 226 which has another of its signal inputs tied to the set output of flipflop 208. The third signal input of gate 226 is tied to line 142 which connects to vehicle detector D9 (FIG. 2). Thus, when gate 224 is providing an output on line 139R to cause traflic signal 24 to indicate red, an enabling input is applied from gate 224 to the first signal input of gate 226. If a vehicle is waiting within the detection zone of vehicle detector D9, an enabling input is applied to the second input of gate 226. When the voltage on integrator 134 reaches the firing level of Schmitt trigger 204, fiipflop 208 assumes its set condition, and a signal passes from gate 226 through OR gate 230 to trigger one shot multivibrator 232. The output of one shot 232 causes flipfiop 218 to assume its reset condition and causes flipflop 222 to assume its set condition. As a consequence the set output from fiipflop 218 is terminated, and so the signal on line 139R from gate 224 is no longer provided to cause the red indication from trafiic signal 24. The set output of flipfiop 222 passes through OR gate 234 to line 139G which applies it to traflic signal 24 to cause a green indication, signalling the waiting driver to proceed on entrance ramp 16 to the merging area 20.

The set output of fiipflop 222 passes through OR gate 236 to trigger one shot multivibrator 238 which resets fiipfiop 208. The set output of fiipfiop 222 is also applied to the input of timer 240 which times the duration of the green indication. After times 240 has timed out, it triggers one shot multivibrator 242 which then resets flipflop 222 and sets flipflop 220. When flipflop 222 returns to its reset condition, the signal on line 139G ends, terminating the green indication from traffic signal 24. The set output from fiipfiop 220 is applied via line 139Y to traffic signal 24 to cause the traffic signal to provide a yellow indication to vehicles on entrance ramp 16. Lines 139R, 139G, and 139Y from signal controller 136 to traffic signal 24 thus comprise line 139 of FIG. 2. The set output of flipflop 220 is also applied to the input of timer 244 which times the duratoin of the yellow indication. When timer 244 times out, it triggers one shot multivibrator 246 which resets fiipflop 220 and sets fiipfiop 218. Termination of the set output of flipflop 220 terminates the signal on line 139Y, ending the yellow indication from trafiic signal 24. The set output of fiipflop 218 passes through gate 224 to line 139R to cause trafiic signal 24 to again provide a red indication to vehicles on entrance ramp 16. Thus, when a vehicle is waiting within the detection zone of vehicle detector D9 on entrance ramp 16, the traffic signal 24 provides a green indication to that vehicle as soon as the voltage on integrator 134 reaches a level which indicates that there is sufficient available capacity on roadway 10 to accommodate that waiting vehicle, as determined by the triggering level of Schmitt trigger 204.

If there is no vehicle within the detection zone of vehicle detector D9 at the time integrator 134 output fires Schmitt trigger 204 to set flipfiop 208, gate 226 is blocked, and so the red indication is maintained at traffic signal 24. Continued traflic movement on roadway 10 varies the available capacity on the roadway. The available capacity which resulted in the setting of fiipfiop 208 soon moves down roadway 10 with the traffic movement. Thus, after some period of time that capacity is no longer available at merging area 20, although it still exists downstream. If a vehicle approaches trafiic signal 24 on ramp 16 after this free capacity on roadway 10 is no longer available at merging area 20, it might not be safe for that vehicle to attempt to merge into the trafiic on roadway 10. Therefore, the fiipflop 208 set output is applied as the input to timer 248 which times a maximum period during which signal controller 136 is permitted to allow traffic signal 24 to change from red to green in response to the presence of a vehicle within the detection zone of the vehicle detector D9, for example in the range of three seconds. At the end of this maximum time, timer 248 applies a signal through OR gate 236 to trigger one shot multivibrator 2.38 which then resets flipfiop 2%.

When fiipflop 208 is set by the voltage from integrator 134, its set output inhibits gate 2172 and causes relay 214 to reset integrator 134. If no vehicle is within the detection zone of vehicle detector D9, then gate 226 blocks the set output of flipflop 203 from reaching one shot 232. Integrator 134 again commences to generate an output signal indicative of the accumulated available capacity on roadway 10. If the traffic on roadway is very light, the integrator .134 output again reaches the point at which it indicates that there is suflicient available capacity for a vehicle to merge onto roadway 1t). Since gate 202 is blocked, the integrator output does not pass, and the integrator is not reset. Thus, this output condition is maintained. If this condition has been reached by the time that timer 248 causes flipflop 208 to be reset, then termination of the flipfiop set output permits the integrator output to pass through gate 292 to again fire Schmitt trigger 204. This causes one shot 2% to again set flipfiop 2%, and so the signal controller is again able to cause a green indication from traffic signal 24 in response to any vehicle which then comes within the detection zone of vehicle detector D9.

However, if traific on roadway 10 is congested at the time flipflop 208 is set, resetting integrator 134, then before a vehicle comes within the detection zone of detector D2, the integrator output may go in the opposite direction, indicating that the available capacity on roadway 10 no longer exists and instead there is a shortage of capacity. Should this happen, Schmitt trigger 25% turns on, and its output passes through OR gate 236 to fire one shot 238. The one shot output resets flipflop 268, and so signal controller 136 is prevented from causing a green indication on traffic signal 24 in response to any vehicle then coming within detection zone D9. The level of the shortage of capacity which is required to cause fiipflop 2% to be reset is determined by the adjustment of Schmitt trigger 251 As an example of the controller operation and without limitation, assume that the maximum capacity of roadway 1t) is 6000 vehicles per hour (2000 vehicles per hour per lane) and that the congestion-indicating parameters which are applied as inputs to summing amplifier 132 reduce this to an instantaneous effective roadway capacity of 5100 vehicle per hour, and assume that the instantaneous traflic volume demand passing vehicle detectors D1D.3 is 4700 vehicles per hour. Then at that instant there is sufiicient available capacity on roadway 10 to accommodate additional vehicles at a rate equivalent to 400 vehicles per hour, or one vehicle every 9 seconds. Thus, under these conditions, approximately 9 seconds is required for the output of integrator 134 to reach the firing level of Schmitt trigger 204. As the instantaneous effective roadway capacity or the instantaneous traffic volume demand vary, the allowable entrance rate varies, and so the length of time required for the output of integrator 134 to reach the firing level of Schmitt trigger 204 varies accordingly.

If the trafiic on roadway 10 has reached such a level of congestion that the average trafiic speed all along the roadway is low, considerable time may elapse before the integrator 134; output indicates that there is sufficient available capacity on roadway 10 for another vehicle from ramp 16 to easily merge onto roadway 10. During this time several vehicles may enter ramp 16, forming a line in front of traffic signal 24. If too much time elapses before a vehicle is permitted to pass traific signal 24, this 16 line of Waiting vehicles may extend to frontage road 18 and may interfere with normal traffic flow on that road. During this low speed condition on roadway 1t) it frequently is possible for a vehicle to merge onto the roadway even though the indicated available capacity of the roadway is below the level normally required to permit a vehicle to merge. This level of traffic congestion on roadway 10 can be determined from the average traffic speed at vehicle detector D7. When this speed is within a critical range, as determined by speed computer 34-, level monitor 42 applies a signal on line 60 through switch 149 in its closed position to signal controller 136. Within controller 136 this signal on line 6% passes through OR gate 252 to the inhibit input of gate 226. This prevents the output of flip-flop 208 from passing to one shot 232. As a consequence the red indication at control signal 24 cannot be terminated by the output of integrator 134. The signal on line 60 is also applied to the first input of AND gate 254 within controller 136. The set output of flipfiop 218 passes through gate 224 and is applied as an input to gate 254. In addition line 142 connects vehicle detector D9 to an input of gate 254. Thus, when trafiic signal 24- is providing a red indication and level monitor 42 is applying a signal on line 60, then so long as there is a vehicle within the detection zone of vehicle detector D9, gate 254 applies a signal to the input of timer 256. After timer 255 has timed out, it applies a signal through OR gate 231? to trigger one shot multivibrator 232. This resets flipflop 2118, thereby terminating the red indication on traific signal 24, and it sets flipflop 222 initiating the green indication from the trafiic signal. The green indication lasts for a time determined by the adjustment of timer 24 for example a time in the order of one second. Following this time, one shot 242 is triggered to reset fiipfiop 222 and to set fiipfiop 220, thereby providing the yellow indication from trafiic signal 24. The yellow indication lasts for a time determined by the adjustment of timer 244, which for example may be in the order of one second, following which one shot 246 resets flipfiop 224i and sets fiipfiop 218. As a consequence the yellow indication ends, and the red indication is again provided. This red indication is ap plied to gate 254, and if the enabling signal is still present on line 6th and if another vehicle is waiting in front of the control signal 24, gate 254 turns on timer 256 which times a minimum red duration before again causing the green signal to be initiated. This minimum red duration may be in the order of three seconds, for example. Thus, so long as this condition of moderate congestion exists on roadway 10, as indicated by the signal on line 69, and so long as vehicles are within the detection zone of vehicle detector D9, signal controller 126 causes the trafiic signal 24 to cycle through its red-green-yellow sequence at a rate determined by the adjustments of timers 240, 244, and 256.

If desired, to insure that vehicles waiting to pass traffic signal 24 do not form a line passing through left-turn area 22 which interferes with traffic on frontage road 18, a vehicle detector can be placed over or in left-turn area 22 to provide an indication of the presence of vehicles within the left-turn area. Connection of this detector output to a timer then results in a signal from the timer when a vehicle has been in left-turn area 22 for a time long enough to indicate that the vehicle has stopped within the left-turn area. This timer output and the line 69 output of level monitor 42 are then joined via an OR gate and are applied to OR gate 252 and to AND gate 254, as is line 60 in FIG. 4. The signal from the detector in left turn area 22 then insures that trafiic signal 24 cycles through its red-green-yellow sequence at a fixed rate whenever the line of vehicles on ramp 24 reaches left-turn area 22.

Extremely great congestion on the roadway 1% causes very low traffic speeds past the vehicle detectors D1D6. This congestion is first indicated by the speed output from downstream detector D5. When the average trafiic speed past downstream vehicle detector D is at a very low level, level monitor 40 applies a signal via line 55 through switch 140 in its closed position to selector 56. This signal causes selector 56 to disconnect the output of summing amplifier 132 from the input of integrator 134 and in its place to connect the output of summing amplifier 100 to the input of integrator "134 via line 101. Selector 56 by way of example could be a double-throw relay. Alternatively it could be a solid state switching device. The output of inverter 100 is a voltage representing the total downstream traffic volume passing vehicle detectors D4D6. Thus, the inputs to summing integrator 134 are the downstream traffic volume and the negative of the upstream traffic volume, and so the output of integrator 134 reflects the time integral of the difference between the downstream traffic volume and the upstream trafiic volume. When this time integral reaches a level that indicates that there is available capacity for another vehicle on roadway 10, then signal controller 136 causes traffic signal 24 to provide a green indication, informing a waiting driver on entrance ramp 16 that he should merge onto roadway 10.

As an alternative, selector 56 can be placed in the output line from scaling unit 116 so that with no signal from level monitor 40 to selector 56 the output of scaling unit 116 is modified at summing amplifier 132 by the road way congestion-indicating parameters received through switches 118130, and this signal is applied through resistor 133 to the input of integrator 134. Then with a signal from level monitor 40 to selector 56, the congestion-indicating parameters from switches 118-130 modify the output of inverter 100 at summing amplifier 132, and this signal is applied through resistor 133 to the input of integrator 134. Then under congested traffic conditions the output of integrator 134 represents the time integral of the difference between the downstream traffic volume, as modified by the roadWay-congestion-indicating parameters, and the upstream traflic volume.

Should a sudden condition of extreme congestion occur downstream of ramp 16, for example due to an accident blocking one or more of the lanes of roadway 10, the volume of trafiic passing the downstream vehicle detectors D4-D6 drops to a very low value, but the volume of traffic passing the upstream detectors does not drop so soon. This difference between downstream trafiic volume and upstream trafiic volume is indicated at the output of summing amplifier 102. When this volume difference reaches a critical level, indicative of a blockage of road-way downstream of ramp 16, level monitor 104 applies a signal on line 110 through switch 148 in its closed position to signal controller 136. Within controller 136 this signal on line 110 passes through OR gate 252 to the inhibit input of gate 226. This inhibits the normal operation of controller 136 by keeping the fiipflop 208 set output from reaching one shot 232, as explained above with reference to the signal on line 60. However, since the signal on line 110 is not applied to gate 254, controller 136 does not cycle at a fixed rate, as is the case when a signal is present on line 60. Thus, the red indication is maintained at traffic signal 24 during this critical condition.

If a vehicle has passed control signal 24 on ramp 16 and has entered merging area 20 but has not merged onto roadway 10, then it would be unsafe for another vehicle on ramp 16 to enter the merging area at a high speed. So long as the first vehicle is within the detection zone of vehicle detector D8 an output is applied from detector D8 to timer 76. If that vehicle remains within the detection zone of vehicle detector D8 for a substantial period of time, it indicates that the vehicle has stopped within the merging area, and so timer 76 causes switching unit 78 to apply a signal through switch 144 in its closed position to line 146 which conducts the signal to OR gate 252 within signal controller 136. From OR gate 18 252 the signal is applied to the inhibit input of gate 226 to inhibit the normal controller operation and to retain the red indication at trafiic signal 24, just as does a signal on line 146.

If the traffic on roadway 10 is very light then it is not necessary to control the entrance of vehicles onto roadway 10 from ramp 16. This light trafiic on roadway 10 results in high speed of vehicles passing vehicle detector D7. As a consequence speed computer 34 causes level monitor 42 to apply a signal on its output line 62. This signal on line 62 passes through switch in its closed position to signal controller 136. Within controller 136 the signal on line 62 is applied to the inhibit input of INHIBITED-AND gate 224, thereby preventing the signalling of the red indication from traific signal 24.

The signal on line 62 also passes through OR gate 234' to cause the green indication from trafiic signal 24. This condition results in a continuous green signal for vehicles entering roadway 10 from entrance ramp 16. This same signal condition can, of course, be obtained by other logic.

It can thus be seen that the expressway ramp traflic control system of the present invention is capable of ad justing the rate at which vehicles are signalled to enter such a roadway to fit the available capacity and in accordance with the degree of congestion on that roadway, as determined by the operation of the traffic already on the roadway. The above description of the preferred embodiment of the invention has disclosed use of traflic speed and traflic volume characteristic to determine roadway congestion. Obviously, other characteristics such as trafi-ic density or percentage occupancy or any combination of all of these characteristics could be utilized to propide the parameters from which is determined the amount of available capacity existing on the limited access roadway.

Operation of the present system can be adapted to meet the particular requirements of any expressway entrance ramp by selectively opening one or more of the switches 118-130. Opening one or more of these switches results in the congestion-indicating parameters associated with the open switches no longer influencing the indicated roadway capacity. In addition, the elfect of ex tremely light trafiic or of highly congested traflic conditions can be removed from the system operation by opening one or more of the switches 140, 144, 148, 149 and 150.

While particular detector locations or combinations of detector locations have been disclosed for controlling particular congestion-indicating parameters, obviously alternative detector locations might be utilized in place of some of these. For example, while low average trafiic speed passing vehicle detector D5 and high average speed passing vehicle detector D2 have been disclosed as one indication of traffic congestion, and this indication has been used as inputs to AND gate 52, obviously, the combination of low trafiic speed passing vehicle detector D7 and high traffic speed passing vehicle detector D2 or the combination of low trafiic speed passing the detector D5 and high average trafiic speed passing detector D7 could be utilized to provide this indication. This simply requires changing the inputs applied to gate 52. Additionally, while particular speed combinations have been disclosed for generating the congestion-indicating parameters, obviously these are only representative, and other combinations could be utilized by adjusting the level monitors as desired. Thus, for example, AND gate 52 could turn on timer 72 when level monitors 38 and 40 indicate low traffic speeds past both vehicle detector D2 and vehicle detector D5.

Vehicle detector D9 could be omitted, and the requirement for its input removed from gate 226. Then traffic signal 24 provides a green indication each time the output of integrator 134 reaches the firing level of Schmitt trigger 204, so long as the red indication is present on 19 line 139R and so long as no signal is applied to gate 226 from OR gate 252.

Another vehicle detector could be placed on ramp 16 just beyond traffic signal 24 with its output applied to the input of one shot 242, along with that of timer 240 via an OR gate. Then vehicles passing over this detector terminate the green indication, and timer 240 provides a maximum green duration and terminates the green in the event the waiting driver hesitates so long a time that the available capacity on roadway has moved past merging area 20, for example a maximum time of three seconds. Since only one vehicle at a timep asses traffic signal 24, the yellow or clearance indication can be omitted, with traffic signal 24 only providing red and green indications. This can be accomplished by setting timer 244 to zero seconds or by applying the one shot 242 output to the set input of fiipfiop 218 rather than flipflop 220.

Under extremely light trafiic conditions, it might be desirable to provide no indication at all from traffic signal 24. This can be accomplished by simply removing the input of line 62 to OR gate 234. Then, when the signal on line 62 indicates that extremelylight traffic conditions exist, gate 224 is inhibited so that the red indication is not provided from traflic signal 24, and no other indication is provided in its place. Vehicles utilizing entrance ramp 16 then proceed at the discretion of their drivers.

While particular voltage values and voltage polarities have been suggested, these of course, are only illustrative. Other voltage values might be utilized, and opposite voltage polarity might be utilized by making appropriate changes within the detailed circuitry.

The sensitivity of the trafiic control system to changes in traffic conditions on roadway 1.0 can be controlled in several places. The length of time over which speed computers 32-36 and volume computers 84-94 average their inputs can be adjusted, thus causing a slower or a faster response to surges of traffic. The level monitors 38-44 and 104 can be adjusted to respond to different input levels, as can the Schmitt trigger circuits. The input and feedback resistors to the summing amplifier 96-102 and the input resistors 133 and 133' and the feedback capacitor 135 of integrator 134 can be varied to vary the eifect of difierent input signals. Obviously numerous other means of controlling the circuit sensitivity can also be utilized.

The available capacity is distributed between all three lanes of roadway 10 and so is not all available in the right-hand lane, nearest merging area 20. To some extent vehicles in the center and right-hand lane of roadway 10 will move to the left-hand and center lanes respectively, thereby resulting in more of the available capacity being in the right-hand lane, particularly under light traflic conditions. The effect on the total indicated available capacity which is due to the available capacity in the lanes more distant from merging area 20 can be weighted by weighting the values of the input resistors to summing amplifier 96 associated with more distant lanes.

Although preferred embodiments of particular components of the traffic control system have been described, these are illustrative examples only and they are not intended as limitations. Other equivalent devices can be used in place of one or more of these components and still be within the present invention.

The invention has been discussed with reference to a limited access roadway and an entrance ramp. It is, of course, obvious that it can be utilized in controlling traflic entering any heavily travelled main roadway from a comparatively lightly travelled side roadway. Thus it can be utilized at the intersection of an arterial highway and a merging side street or at a merging access road on a bridge or in a tunnel. In addition, while trafiic volumes in all three lanes of roadway 10 are monitored by vehicle detectors D1-D6, it would be possible to provide a system monitoring only the trafiic volume in the right-hand lane of the roadway. In such a system, vehicle detectors D1 and D4 would provide both trafiic volume and traffic speed information. Then vehicle detectors D2, D3, D5, and D6 would not be required. Even utilizing six vehicle detectors to monitor all three lanes of roadway 10, speed in a lane other than the center lane could be monitored.

While particular logic combinations are disclosed in the above embodiment, obviously inverting amplifiers, isolating amplifiers, isolating diodes, delay multvibrators, and other minor components can be added as required. While positive logic has been shown, with appropriate modification negative logic can be utilized in the logic circuitry. Should it be desired to have a minimum duration of the red indication from traflic signal 24 during normally-congested traffic conditions, a timer adjusted for this minimum duration can be inserted in the line coupling the output of gate 224 to the input of gate 226. By proper design, certain of the summing amplifiers might be eliminated, but they have been shown in the above preferred embodiment in the interest of clarity. To accommodate the current required by the trafiic signal 24 lamp loads, lines 139R, 139G, and 139Y can control external relays which switch current to the signal lamps.

Thus, among others, the several objects of the invention, as specifically aforenoted, are achieved. Although alternative forms of the invention have been pointed out above, obviously numerous other changes in construction and rearrangement might be resorted to without departing from the spirit of the invention.

What is claimed is: 1. Apparatus for controlling entry of vehicles into traffic flow on a main road way from a side roadway which terminates in a merging area with the main roadway, said apparatus comprising in combination:

first circuit means responsive to at least one characteristic of traffic flow on said main road-way for providing a first signal indicative of the traffic capacity of said main roadway, said first signal being determined from a plurality of roadway-congrestion-indicative parameters derived from said at least one characteristic; second circuit means for providing a second signal indicative of the trafiic demand on said main roadthird circuit means coupled to said first and second circuit means for generating a third signal in response to a comparison between said first and second signals, said third circuit means including integrating means for generating the time integral of the difference between the traffic capacity of said main roadway and the traffic demand on said main roadway; and

fourth circuit means coupled to said third circuit means for signalling to a waiting vehicle on said side roadway to enter said main roadway traflic flow when said third signal indicates the presence of sufficient available capacity in excess of demand on said main roadway to accommodate a vehicle from said side roadway.

2. Apparatus as claimed in claim 1 in which said first circuit means includes:

vehicle detection means for providing indications of the presence of vehicles on said main road-way, said side roadway, and said merging area;

congestion indicating means coupled to said vehicle detection means for generating a plurality of signals indicative of traffic congestion on said main roadway in response to a plurality of roadway-congestionindicative parameters derived from said vehicle presence indications;

means for providing a source of voltage representing the maximum capacity of said main roadway; and voltage summing means coupled to said voltage source 21 means and to said congestion indicating means for providing an output representing main roadway trafiic capacity as indicated by said maximum capacity and said traffic congestion indicative signals. 3. Apparatus as claimed in claim 2 in which said fourth circuit means is further coupled to said vehicle detection means and in which said fourth circuit means includes:

storage means for storing a permissive-condition indication when said time integral reaches a preset level;

means for resetting said storage means after said permissive-condition indication has been stored for a preset time;

means for causing said fourth circuit means to signal in response to an indication from said vehicle detection means of the presence on said side roadway of a vehicle waiting to enter said main roadway trafiic flow during the time said permissive-condition indication is stored; and

means for resetting said storage means when said fourth circuit means signals.

4. Apparatus for controlling entry of vehicles into tratfic flow on a main roadway from a side roadway which terminates in a merging area with the main roadway, said apparatus comprising in combination:

first circuit means responsive to at least one characteristic of traffic flow on said main roadway for deriving a first signal indicative of the traffic capacity of said main roadway;

second circuit means for providing a second signal indicative of the traffic demand on said main roadway; third circuit means coupled to said first and second circuit means for generating a third signal in response to a comparison between said first and second signals;

fourth circuit means coupled to said third circuit means for generating a permissive-condition indication when said third signal indicates the presence on said main roadway of suflicient available capacity to accommodate a vehicle from said side roadway;

storage means for storing said permissive-condition indication;

means for resettting said storage means after said stored permissive-condition indication has been present for a preset time;

sensing means for sensing on said side roadway the presence of a vehicle waiting to enter said main roadway traffic flow;

fifth circuit means coupled to said storage means and to said sensing means for signalling to a waiting vehicle on said side roadway to enter said main roadway traific flo'w in response to an indication from said sensing means of the presence on said side roadway of a vehicle waiting to enter said main roadway tralfic flow during the presence of said stored permissive-condition indication; and

means for resetting said storage means when said fifth circuit means signals.

5. Apparatus as claimed in claim 4 further comprising means for resetting said storage means when said third signal indicates a shortage of capacity on said main roadway of at least a preset level.

6. Apparatus as claimed in claim 4 in which said first circuit means includes:

vehicle detection means for providing indications of the presence of vehicles within selected detection zones on said main roadway, said side roadway, and said merging area,

speed computing means coupled to said vehicle detection means for providing speed signals of varying levels indicative of the average speed of vehicles passing through selected ones of said detections zones,

level sensing means coupled to said speed computing means for providing selected ones of a plurality of output conditions in response to said speed signals, time-controlled switching means selectively couple to said level sensing means for providing signals representing main roadway congestion-indicative parameters in response to selected level sensing means output conditions having preset minimum time durations. means for providing a source of voltage representing the maximum capacity of said main roadway, and summing means coupled to said voltage source means and to said switching means for providing a signal representing the available; main roadway traffic capacity as indicated by the algebraic sum of said maximum main roadway capacity and said main roadway congestion indicative parameters; and in which said third circuit means includes:

voltage integrating means coupled to said summing means and to said second circuit means for providing the time integral of the difference between said available main roadway traffic capacity and said main roadway traffic demand; and in which said fifth circuit means includes:

traflic signalling means adjacent said side roadway and capable of alternatively providing a proceed signal and a stop signal to vehicles on said side roadway waiting to enter said main roadway trafiic flow, signal cycling means coupled to said traffic signalling means, and voltage gating means coupled to said sensing means, said storage means, and said signal cycling means for applying an activating signal to said signal cycling means in response to the presence of a vehicle on the entrance ramp during the presence of said stored permissive-condition indication, said signal cycling means upon application of said activating signal causing said traffic signalling means to provide a proceed signal of a preset maximum duration and subsequently to provide a stop signal. 7. Apparatus for controlling the entry of vehicles into trafiic flow on a main roadway from a side roadway which terminates in a merging area With the main roadway, said apparatus used with vehicle detection means of the type which provides an indication of vehicle presence and used with traffic signalling means of the type which provides a proceed indication in response toan activating signal, said apparatus comprising in combination:

first circuit means connected to vehicle. detection means for providing a first signal indicative of the tratfic capacity of said main roadway, said first circuit means including congestion indicating means connected to said vehicle detection means for generating a plurality of signals indicative of traffic congestion on said main roadway in response to a plurality of roadway-congestion-indicative parameters derived from vehicle presence indications, means for providing a source of voltage representing the maximum capacity of said main roadway, and voltage summing means coupled to said voltage source means and to said congestion indicating means for providing an output representing main roadway traffic capacity as indicated by said maximum capacity and said traffic congestion indicative signals; second circuit means connected to vehicle detection means for providing a second signal indicative of the traffic demand on said main roadway; third circuit means coupled to said first and second circuit means for providing a third signal in response to a comparison between said first and second signals; fourth circuit means connected to said third circuit means and connected to tratfic signalling means for applying an activating signal to said trafiic signalling means when said third signal indicates the presence on said main roadway of suflicient available capacity in excess of demand to accommodate a vehicle from said side roadway, said fourth circuit means connected to vehicle detection means and including storage means for storing a permissive-condition indication in response to an indication by said third signal of the presence on said main roadway of sufficient available capacity in excess of demand to accommodate a vehicle from said side roadway, means for resetting said storage means after said permissivecondition indication has been stored for a preset time, means connected to said vehicle detection means for applying an activating signal to said trafiic signalling means in response to an indication from said vehicle detection means of the presence on said side roadway of a vehicle waiting to enter said main roadway traffic flow during the time said permissive-condition indication is stored, and means for resetting said storage means in response to said activating signal. 8. Apparatus for controlling the entry of vehicles'into traffic flow on a main roadway from a side roadway which terminates in a merging area with the main roadway, said apparatus used with vehicle detection means of the type which provides an indication of vehicle presence and used with traffic signalling means of the type which provides a proceed indication in response to an activating signal, said apparatus comprising in combination:

first circuit means connected to vehicle detection means for deriving from vehicle presence indications a first signal indicative of the trafiic capacity of said main roadway; second'circuit means connected to vehicle detection means for providing a second signal indicative of the traffic demand on said main roadway; third circuit means coupled to said first and second circuit means for generating a third signal in response to a comparison between said first and second signals;

24 fourth circuit means coupled to said third circuit means for generating a permissive-condition indication when said third signal indicates a presence on said main roadway of sufficient available capacity to accom modate. a vehicle from said side roadway; storage means connected to said fourth circuit means for storing said permissive-condition indication;

means for resetting said storage means after said stored permissive-condition indication has been present for a preset time; and

fifth circuit means connected to said storage means and connected to vehicle detection means and to traffic signalling means for generating an activating signal for said traffic signalling means in response to a vehicle presence indication from said vehicle detection means, during the presence of said stored permissive-condition indication, and for resetting said storage means upon generation of said activating signal.

9. Apparatus as claimed in claim 8 further comprising means for resetting said storage means when said third signal indicates a shortage of capacity on said main road Way of at least a preset level.

References Cited UNITED STATES PATENTS 3,304,539 2/1967 Aver 34036 3,333,240 7/1967 Gerlougl et a1. 34-O--36 3,384,869 5/1968 Waldrorr 34036 THOMAS B. HABECKER, Primary Examiner C. MARMELSTEIN, Assistant Examiner US. Cl. X.R.

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