US3784971A

US3784971A - Digital system for controlling traffic signals

Info

Publication number: US3784971A
Application number: US00226919A
Authority: US
Inventors: J May
Original assignee: Individual
Current assignee: Individual
Priority date: 1972-02-16
Filing date: 1972-02-16
Publication date: 1974-01-08
Anticipated expiration: 1991-01-08

Abstract

System for controlling traffic signals in which a local controller is located at each controlled intersection, respectively, to control the operation of a traffic signal at its associated intersection. A central controller is connected to the local controllers for the transmission of signals thereto. The central controller is programmed to provide output data which activate the local controllers. The data includes two digital words. One digital word selects the local controller to be activated and the other digital word selects the phase to which the selected local controller is to be advanced. The selected local controllers are operated by the central controller for controlling the traffic lights at their associated intersections. Each local controller includes a set of timers for controlling intervals that are program invariants. The local controllers will continue to operate in a fixed time mode in the absence of an overriding signal for the local controllers. When a local controller is activated by the central controller to advance to a designated light phase, the local controller will advance to the appropriate vehicle clearance interval and to time-out this interval independently before advancing the intersection lights to the phase designated by the central controller.

Description

United States Pate May [ Jan. 8, 1974 DIGITAL SYSTEM FOR CONTROLLING TRAFFIC SIGNALS [76] Inventor: John J. May, 1398 Flicker Way,

Sunnyvale, Calif. 94087 [22] Filed: Feb. 16, 1972 [21] Appl. No.: 226,919

Primary ExaminerWilliam C. Cooper Armrney.lack M. Wiseman [57] ABSTRACT System for controlling traffic signals in which a local controller is located at each controlled intersection, respectively, to control the operation of a traffic signal at its associated intersection. A central controller is connected to the local controllers for the transmission of signals thereto. The central controller is programmed to provide output data which activate the local controllers. The data includes two digital words. One digital word selects the local controller to be activated and the other digital word selects the phase to which the selected local controller is to be advanced. The selected local controllers are operated by the central controller for controlling the traffic lights at their associated intersections. Each local controller includes a set of timers for controlling intervals that are program invariants. The local controllers will continue to operate in a fixed time mode in the absence of an overriding signal for the local controllers. When a local controller is activated by the central controller to advance to a designated light phase, the local controller will advance to the appropriate vehicle clearance interval and to time-out this interval indepen dently before advancing the intersection lights to the phase designated by the central controller.

2 Claims, 11 Drawing Figures PAIENTEDJAH 8 I974 SHEUEOFQ FIGRE 3d FIGURE 2 mm. mm, PM

FIGURE 3 HM F.G.3d

ma mb We PAIENIED 8W4 3,784.97].

sum 3 0f 9 FiGURE 20 FRFQUEA/CY DIV/DER PULJ'E GENERATOR D-FL/P FLOP TO PROGRAM SELECT/N6 CIRCUIT,

PROGRAM MEMO/FY C/RCU/TJ 0CODER PATENTEU 81974 3,784,971

. SHEEI 4 [IF 9 FIGUR 2b PATENTED 81974 3784971 SHEET 5 [1F 9 FIGU RE 2C OUTPUT BUF 5O 51 2% To local 5 controllers PATENTEU 81974 SHEET 7 [1F 9 T FIGURE COUNTING CIRCUIT COUNTING CIRCUIT COUNT/NG \J CIRCUIT Ill,

COUN rk/ve CIRCUIT PMEMEB 81814 I 3,784,971

Sllliii l 8 OF 9 lGE 3C LA TCH CIRCUIT m/pur BUFFR DECODER PAIENTEU JAN 81974 SHEET 9 OF 9 FIGURE 3e 2 RED F 205 2 210 i b; VELLOWI RED 212 I F.- k :GREENI 213 l )w I v YELLOWI RED 2143 G/iL-ENI P215 7 {rEzLowl RED 216 HGREENI 217 1 L rsuozl A PHASE 8 PHASE C PHASE D PHASE DIGITAL SYSTEM FOR CONTROLLING TRAFFIC SIGNALS BACKGROUND OF THE INVENTION The present invention relates in general to apparatus for controlling traffic signal, and more particularly to apparatus for controlling the phase of a plurality of traffic signals.

Heretofore, each controlled intersection was individually controlled by its individual local controller. Each local controller had its own timing arrangement. Also, a central digital computer operated a plurality of local controllers. To coordinate a series of spaced intersections along a thoroughfare or a series of spaced intersections within a prescribed area, an operator would manually adjust each local controller indivudually or would operate all the local controllers by a central digital computer. However, such systems were not capable of skipping phases and regardless of computer output each local controller must follow the predetermined sequence.

An article of interest is Contemporary Concepts of Microprogramming and Emulation" by Robert F. Rosin appearing in Computer Surveys, Vol. 1, No. 4, December 1969, and a book entitled Microprogramming" by Husson. Also of interest is a report by Stanford Research Institute dated October 1971 entitled Chicago CBD Traffic Control System. Multi Sonics Development Corporation of Alamo, Calif. has manufactured and sold Model FR-200 On-Line Master Computer for traffic control, and FR-IOO Traffic Controllers. Gulf-Western Industries at Davenport, Iowa has manufactured and sold MC Lines Controllers for digital coordination or pretimed control.

SUMMARY OF THE INVENTION A system for controlling the operation of a plurality of traffic lights in which a central controller transmits signals to local controllers for selecting one or more local controllers for activation and for preselecting a phase at which the selected local controllers would operate their associated traffic lights.

A feature of the present invention is that the selected local controllers can skip or omit any phase upon command from the central controller. In the absence of a command from the central controller the local controllers, respectively, will operate at preset fixed times modes.

By virtue of the present invention, traffic signals along a thoroughfare or within a preselected area can be automatically coordinated from a central station to effect an optimum flow of traffic through continuous movement of vehicles at an optimum velocity at greatly reduced circuit complexities.

Another feature of the present invention is that the central controller is programmed with a plurality of predetermined programs. Data transmitted by the central controller to the local controllers comprises a series of signals to select one or more of the local controllers to be activated and also comprises a series of signals to select the phase at which the selected local controllers advance to operate their respective associated traffic lights. A local controller may be activated by the central controller and the local controller may be locally controlled for a preselected fixed time operation.

DESCRIPTION OF THE DRAWINGS FIG. 1 is a block diagram of a system for controlling traffic signals embodying the present invention.

FIGS. 2a-2c when placed in the manner shown in FIG. 2 is a schematic diagram of the central controller employed in the system shown in FIG. 1.

FIGS. 3a-3e when placed in the member shown in FIG. 3 is a schematic diagram of a local controller employed in the system shown in FIG. 1.

DESCRIPTION OF THE PREFERRED EMBODIMENTS General Description Illustrated in FIG. 1 is a system 10 for controlling traffic signals embodying the present invention. The system 10 comprises a central station 15, which includes a central controller 20 (FIGS. 2a-2c), and a plurality of local stations 21-28. EAch of the local stations 21-28 include a local controller 200 and intersection traffic lights. In FIGS. 3a-3e is illustrated the local controller 200. All local controllers 200 include like parts and operate in a similar manner. Suitable conductors 50-57 carry signals from the central controller 20. Each local station 21-28 includes suitable terminals for connection to the conductors 5057 for receiving signals transmitted from the central controller 20. While signals are shown to be transmitted in parallel, it is apparent that the transmission may be effected by serial means. This would entail the addition of appropriate converters.

The central controller 20 is programmed with preselected data stores in semiconductor devices. An operator or computer selects one of the programs by selecting a memory device and thereby transmits data representing information stored in the selected memory devices over the conductors 5057. The data contained in the selected memory device has two words. The first word activates selected local stations 21-28 and the second word operates the local controllers of the selected local stations for advancing the associated traffic signals to a designated phase.

CENTRAL CONTROLLER CIRCUITS As shown in FIGS. 2a-2c, the central traffic controller 20 comprises a conventional timer or time pulse generator 60 that produces 60Hz clock pulses. The clock pulses are fed to a conventional pulse frequency divider circuit 61. In the exemplary embodiment, the output frequency of the clock pulses for the divider circuit 61 is such that the leading edge ofa clock pulse occurs every 2 seconds and a traffic control cycle occurs every 64 seconds.

The output of the clock pulse divider circuit 61 provides clock signals for three edge-triggered D-type flipflop circuits or bistable binary devices -72. The flipflop circuits 70-72 incorporate therein two binary circuits in each package anc can be implemented by the Texas Instruments SN 7474. The logic data applied to the input side thereof is transferred to its output side upon the positive going transition or leading edge of the clock signal.

The flip-flop circuits 70-72 produce logic levels which are fed into a sequence memory circuit through a memory decoder 75a and also to eight program memory circuits, only four of which are shown, namely: program memory circuits -83. The transmission to the respective program memory circuits 80-83 are through their associated memory decoders 80a-83a. It is to be observed that the program memory circuits 80-83 are connected in parallel to the output of the flip-flop circuits 70-72 as is the sequence memory circuit 75. Therefore, the output of the flip-flop circuits 70-72 are transmitted simultaneously to the sequence memory circuit 75 and all of the program memory circuits, such as 80-83.

The sequence memory circuit 75 is programmed to function as a counter. However, the program may be varied so that the count may be in a numerical sequence as in the Wilkes principle. The sequence memory circuit 75 is conventional and is similar to the 8223 FROM device produced by Signetics Corp. of Sunnyvale, Calif.

The output of the sequence memory circuit 75 provides the address of the next word in the sequence. In addition thereto, the sixth bit of the output of the sequence memory circuit 75 provides a clock input to a program select circuit 85. More specifically, the clock input transfers the logic levels on the input of D-type flip-

flop circuits

90 and 91 in the manner previously described for the flip-flop circuits 70-72. This action inhibits a change in program until the end of a cycle. The input signals for the flip-

flop circuits

90 and 91 may be from any suitable source such as a time clock.

The program selecting apparatus will preset or precondition the flip-

flop circuits

90 and 91 for a prescribed program operation. At the end of each cycle, which is when the last word of the sequence memory circuit 75 is being addressed, the sequence memory circuit 75 emits an output signal to change the state of the flip-

flop circuits

90 and 91. Connected to the output of the flip-

flop circuits

90 and 91 is a conventional 4 to 16 decoder circuit 95.

The output of the decoder 95 includes, in the exemplary embodiment, eight output terminals. The program memory circuits, such as program memory circuits 80-83 are connected respectively to the output terminals of the decoder 95 and the program memory circuits are respectively enabled or disabled by the signals appearing on the associated output terminal on the decoder circuit 95. Thus, for each given program selection only one of the program memory circuits will be operated through the program selector circuit 85. More specifically, for each different program selection a different program memory circuit will be operated. Only one program memory circuit is operated during a given selected program. The decoder 95 is conventional and is procuced by Signetics Corp. of Sunnyvale, Calif. as device No. 74,154 or 74,141. The flip-

flops

90 and 91 with the decoder 95 serve to inhibit the changing of a program in the middle of a cycle and would tend to reduce operation changes from false signals.

The program memory circuits, such as program memory circuits 80-83, receive simultaneously the outorder to activate the associated traffic displays or lights.

The program memory circuits are similar to devices 8223 FROM produced by Signetics Corp. of Sunnyvale, Calif. The output terminals of the program memory circuits 80-83 are connected in the OR configuration to an output buffer 99. Hence, the only data that is presented to the output buffer 99 is the word being addressed in the enabled memory circuit of the memory circuits 80-83. The output buffer 99 may be any suitable signal conditioning device which changes the output signals to render the same compatible with the signal transmission facilities. In the exemplary embodiment, the output signals from the buffer 99 are transmitted in parallel over the conductors 50-57. When a serial converter is employed for series transmission the number of conductors may be reduced.

From the foregoing, it is to be observed that the output of the sequence memory decoder 75a is gated through the flip-flop circuits -72. The output of the flip-flop circuits 70-72 is transmitted to the input of the sequence memory decoder a and to the input sides of the program memory circuits -83. The logic levels produced by selected program memory circuit are transmitted to the output buffer 99, which then transmits the code pulses over the conductors 50-57 to the local controllers 200. At the end of each cycle, the sequence memory circuit 75 operates the flip-flop circuits and 91 of the program select circuit 85.

LOCAL CONTROLLER Illustrated in FIGS. 3a-3e is a local controller 200 which produces clock pulses of an adjustable constant frequency transmitted from a clock pulse generator 230'. A presettable binary counter circuit 202 has clock pulses applied to its pulse terminal 6 which controls the logic on its

output terminals

12, 2 and 9. The counter circuit 202 is an integrated circuit manufactured by Texas Instruments as SN 74,197.

Connected to the output of the counter circuit 202 is a decoder circuit 205 which includes NOR gate circuits 206-209, NAND gate circuits 210-217. The gates 206-217 of the decoder circuit 205 will selectively have an output 1 potential or a zero potential. When the output potential ofa given gate is O or low, its associated lamp is illuminated. When the output potential of a given gate is l or high, its associated lamp is extinguished. For example, if the output at

pins

12, 2 and 9 of the counter circuit 202 were all low (000), then the output potentials for the gate circuits 207-209 are low, and the associated red lamps will be illuminated. Also, the output potential of the gate circuit 206 is high, or at a 1 potential, and the red lamp connected to its output is extinguished. At this time, the output of the

gates

211, 213, 215 and 217 will be high to extinguish the yellow lamps associated therewith. The gate circuit 210 will be low to illuminate the green lamp associated therewith. The traffic lights are well-known and are conventionally operated in a well-known manner through the associated gate circuits 206-217. The traffic lights or displays with the operating circuits thereof are designated red, yellow and green for the respective phases A-D. The NAND gates 210-217 are manufactured by Signetics or Texas Instruments as SN 7400 integrated circuits.

With the output of the NAND gate 210 at zero potential, the input terminal of the counter circuit 220 is at low potential and the input terminal of counter circuits 221-227 are high. Therefore, the counters 221-227 are reset to zero and held in that condition. The counters are conventional counters of the type manufactured by Signetics or Texas Instruments as SN7493. With the input terminal thereof at zero potential the counter 220 is enabled for counting. The frequency divider 230, which has a frequency of one pulse per second, is connected to a .II( flip-flop circuit 231. The clock pulse output from the JK flip-flop 231 is transmitted simultaneously to the counters 220 and 225-227 over a conductor 232. Each time the counter 220 receives a clock pulse from the .IK flip-flop circuit 231, which is once every 2 seconds, the output thereof is advanced by one binary digit. The counters 225-227 are not enabled at this time to count. The .IK flip-flop is manufactured by Signetics or Texas Instruments as a SN 7473 integrated circuit.

Connected to the output circuits of the binary counters 220 and 225-227 are switching networks, 240 and 245-247 and connected to the other side of the switching networks 240 and 245-247 are NAND gate circuits 250 and 255-257. The NAND gate circuits 250 and 255-257 are manufactured by Signetics or Texas Instruments as SN 7420 integrated circuits. It is apparent to one skilled in the art that pull up resistors at the input of the NAND gates 250-257 may be required.

When the counter 220 has advanced to a binary output (l l l l 32 seconds has elapsed. At this time, the NAND gate 250 will have a zero potential on its output, assuming that all switches are closed. This action causes the output of a NAND gate circuit 260 to go high or to a 1 potential. Prior to the NAND gate 250 having a Zero potential at its output, all the input circuits to the NAND gate 260 were high, and also all the input circuits except for the terminal 9 of an AND-OR- INVERT gate 261 were low. The time required for the input of the NAND gate 250 to reach the 1111 state can be varied by the switching network 240. The NAND gate 260 is manufactured by Texas Instruments as SN 7430 integrated circuit and the AND-OR- INVERT gate 261 is also manufactured by Texas Instruments as a SN 7451 integrated circuit.

When the output potential for the NAND gate 260 goes high, the output of the AND-OR-INVERT 261 goes to zero potential. Initially, the input to a NAND gate 262 is high. With the output of the AND-OR- INVERT gate 261 low, the output of the NAND gate 262 goes to a 1 potential. The counter 202 does not count until the clock input at the terminal 6 goes low. This occurs when the input to the NAND gate 250 advances to something other than 1 l l 1.

When the output from the counter 220 goes from a 1111 output to a 0000 output, the NAND gate 250 will have a 1 potential on its output. The output potential of the NAND gate 260 will go low. When the output potential of the NAND gate 260 goes low, the AND- OR-INVERT gate 261 has a 2 potential on its outpout and the outputs of the gates 206 and 210 of the decoder 205 are high so that the A phase green and the A phase red lamps are off. The outputs of the gates 207-209 are low toiIIuminate the red lamps of the 82C and D phases.

With the output of the gate 211 of the decoder 205 low, the terminal 2 of the counter 221 is low. Thereupon the counter 221 is enabled for counting and the counters 222-224 will be held at 0000 output potential state. The clock generator 230 through the frequency divider 230 supplies the clock pulses to operate the counter 221 for a counting cycle. In the counters 221-224 only three bits are used so that each counter can count up to binary 7. The clock pulses transmitted by the clock pulse generator 230 are fed to the NAND gates 251-254.

When the input to the NAND gate 251 reaches 111, the output of the NAND gate 265 goes high. The high output potential of the NAND gate 265 changes the state of the AND-OR-INVERT gate 261 to turn on the NAND gate 262. This action transmits a pulse to the input terminal 6 of the counter circuit 202 to advance the counter 202 to 010 potential output on its

terminals

12, 2 and 9.

The purpose of switching networks 240-247 is to adjust the green light duration for the various phases. The opening of a switch presents a high potential to the associated NAND gate circuit. When the counter output is low, the opening of the switch causes the associated NAND gate circuit to operate as if the counter output is 1 although it may actually be Zero.

The foregoing steps are continued for all of the four phases of operation. Should it be desired to use the local controller 200 as a two or three phase unit, then the input signal to a NAND gate 266 will be connected to the output of the counter circuit 202. In the two phase operation, one of the inputs to the NAND gate circuit 266 would be opened and the other input would be connected to terminal 12 of counter circuit 202.

The A phase green lamp will be illuminated when the potential output from the counter circuit 202 to the decoder 205 is 000. The A phase yellow lamp will be illuminated when the potential output from the counter circuit 202 to the decoder 205 is 001. The B phase green lamp will be illuminated when the potential output from the counter circuit 202 to the decoder 25 is 010 and the B phase yellow lamp will be illuminated when the potential output from the counter circuit 202 to the decoder 205 is 01 1.

At the end of the B phase yellow time sequence, the potential applied to the input of the decoder circuit 205 from the output of the counter circuit 202 is 100. At this time, the input terminals of the NAND gate 266 are at a high potential. The output of the NAND gate 266 goes low to clear the counter 202 and reset the output potential thereof to 000. The clearing and resetting takes less than a microsecond. In a three phase operation, the input of the NAND gate 266 is connected to the

output terminals

12 and 2 of the counter circuit 202.

Should the central controller 20 (FIGS. 2a-2c) transmit signals to the local controller 200 over conductors 50-57 to override the fixed time pulses of the frequency divider 230, then the local controller 200 can be operated to advance to a preselected phase. The output of the central controller 20 will include a code for addressing the intersection and for selecting the phase. For example, A phase would be encoded with 00, B phase with an 01, C phase with a and D phase 1 1.

The local controller 200 includes an input buffer 100 with level changing and signal squaring devices connected to the conductors 50-57. The output of the buffer 100 is connected to a latch circuit 271 and the decoder 101. The output of the decoder 101 provides input for a NAND gate circuit 270 and the J input to a JK flip-flop circuit 272. The latch circuit 271 may be of the type manufactured by Texas Instruments as SN 7475 and the JK flip-flop circuit 272 may be of the type manufactured by Texas Instruments as SN 7473. The output of the NAND gate 270 is connected to the NAND gate circuit 262 and the output of the NAND gate circuit 273 is connected to the counter circuit 202. The logic transmitted from the central controller 20 to the latch circuit 271 are the phase bits of the transmitted code and the logic transmitted to the JK flip-flop 272 from the central controller 20 through the decoder 101 is dependent upon the address output bits for the intersection of the transmitted code pulses.

Assuming that the local controller is illuminating the A phase green lamp and is to be advanced to the C phase green lamp. Under these circumstances, the potential on the input terminal 6 of the latch 271 is low. The latch 271 has the data or potential applied to the input transferred to its output when the clock input to terminal 4 thereof is at a high or 1 potential.

The input buffer 100 provides a 10 to the

terminals

7 and 6 of the latch 271 and the output of the inverter would go high. This would transfer data on the input of the latch 271 to the output thereof at the

pins

9 and 10. At this time, the flip-flop 272 is set and input of the NAND gate 270 goes high.

When the data was transferred to the output of the latch 271, such data was transmitted to the input side of the counter circuit 202. When the output of the NAND gate 273 is low so that the potential applied to the load terminal 1 ofthe counter circuit 202 is low, the input data applied to the

terminals

11, 3 and 10 of the counter 202 are transferred to the

output terminals

12, 2 and 9 of the counter 202. The output of the counter 202 at that time is 100.

The pulse from the central controller 20 that had set the latch 271 also applies a high potential to the .1 input of the JK flip-flop 272. The truth table for the JK flipflop 272 is as described in the Signetics or Texas Instruments data sheet or catalogues.

Applied to the JK flip-flop are clock or timing pulses from the frequency divider 230. At the falling edge of the clock pulse applied to the JK flip-flop 272, the flipflop 272 is set with the 0 high and the 6 low.

If the controller 200 is in a yellow state, the input to the NAND gate 270 from the output of the inverter 208 is low. Consequently, the counter 202 cannot be advanced while the local controller is in the yellow state except when advanced by one of the counters 241-244. Should the controller 200 be in a green state, the input to the NAND gate 270 from the inverter 208 is high permitting the logic from the decoder 101 to activate the NAND gate 262.

When the NAND gate 270 output is low, the potential input of the NAND gate 262 is low. Although the 6 the counter 202 high, the counter circuit 202 advances the count thereof by l at the falling edge of the clock pulse. The falling edge occurs when the potential applied to the input terminal of the latch circuit 271 from the central controller 20 through the buffer and the decoder 101 is zero. At this time, the output of the counter 202 is advanced to a 001 from a 000 and the state advances from an A phase green to an A phase yellow. The counter circuit 221 now is enabled by the NAND gate 211 and starts to count from the pulses transmitted from the clock pulse generator 230. Thus, the counting by the counter circuit 221 is independent of the code transmission by the central controller 20.

At the termination of the prescribed counting pulses of the yellow phase time period, the output of the NAND gate 251 is low and the output of the NAND gate 265 is high. This action applies a high potential to the input terminal of the AND-OR-INVERT gate 261, the input terminal of the NAND gate 273, and K input terminal of the JK flipflop circuit 272. The state of the AND-OR-lNVERT gate 261 is not changed because of the AND condition with the 6 output of the .IK flip-flop circuit 272, which is connected to the input terminal of the AND-OR-INVERT gate 261. The 6 output of the JK flip-flop circuit 272 is low at this time. However, the Q output of the JK flip-flip circuit 272 applies a potential to the input terminal of the NAND gate 273 to cause the output of the NAND gate 273 to be at a low potential. This action causes a low potential to be applied to the count/load terminal 1 of the counter circuit 202, thereby pre-loading the counter circuit 202 with the data applied to the

input terminals

11, 3 and 10 of the counter circuit 202. Now, the counter outpout is 100 or a C phase green.

The output of the NAND gate 265 is also applied to K input of the JK flip-flop 272 so that the 6 output terminal thereof is at a high potential and the 0 output terminal is at a low potential. When the clock input from the pulse generator 230 goes low the local controller is in C phase green and the counter circuit 226 is counting. When the local controller is in C phase green, the output of the NAND gate 214 is at a low potential. The potential is applied to a conductor 281 and thereby to the input terminal of the counter 226 to enable the counter circuit 266 to count. Counting pulses are transmitted from the clock pulse generator 230 through the JK flip-flop circuit 231 to the clock input terminal of the counter 226.

I claim:

1. A traffic control system comprising:

a. a central control unit, said central control unit comprising means for transmitting respectively a plurality of selectable programs;

b. a plurality of local control units;

c. a plurality of traffic control displays connected to each of said local control units, said traffic control displays being operatively controlled by the local control unit connected thereto,

d. data of the selected program transmitted by said central control unit to said local control units selecting at least one of said local control units to be operated;

e. each of said local control units comprising timing means to provide signals for controlling the operation of the traffic control displays connected to the local control unit; and

10 wherein said selected control unit comprises circuit means for disabling the timing means thereof in controlling the operation of the traffic control displays connected thereto in response to said central control unit providing therefor a signal for controlling the operation of the traffic control displays connected thereto.

Claims

1. A traffic control system comprising: a. a central control unit, said central control unit comprising means for transmitting respectively a plurality of selectable programs; b. a plurality of local control units; c. a plurality of traffic control displays connected to each of said local control units, said traffic control displays being operatively controlled by the local control unit connected thereto, d. data of the selected program transmitted by said central control unit to said local control units selecting at least one of said local control units to be operated; e. each of said local control units comprising timing means to provide signals for controlling the operation of the traffic control displays connected to the local control unit; and f. means in said central control unit for providing data to said selected local control unit to dominate the operation of the selected local control over the signals provided within the selected local control unit for controlling the operation of traffic control displays connected to the selected local control unit.

2. A traffic control system as claimed in claim 1 wherein said selected control unit comprises circuit means for disabling the timing means thereof in controlling the operation of the traffic control displays connected thereto in response to said central control unit providing therefor a signal for controlling the operation of the traffic control displays connected thereto.