GB2425010A

GB2425010A - Means for determining the condition of traffic along a carriageway

Info

Publication number: GB2425010A
Application number: GB0606622A
Authority: GB
Inventors: Richard Diggory Jenkins
Original assignee: AGD Systems Ltd
Current assignee: AGD Systems Ltd
Priority date: 2005-04-02
Filing date: 2006-04-03
Publication date: 2006-10-11
Anticipated expiration: 2026-04-03
Also published as: GB2425010B; GB0506723D0; GB0606622D0

Abstract

A system is disclosed comprising a Doppler sensor 10 directed towards a carriageway to derive measurements of frequency data, representing the number of vehicles passing along the carriageway, and rate data, representing the acceleration or deceleration of the vehicles monitored on the stretch of a carriageway. The frequency and rate data is processed by a state engine (Fig.2) to determine the current traffic condition. The state engine may comprise discrete finite states corresponding to empty, free flowing, traffic slowing, congested, traffic increasing speed and queuing conditions along the carriageway.

Description

I

METHOD AND APPARATUS FOR DETERMINING A TRAFFIC CONDITION

This invention relates to a method and apparatus for determining a traffic condition of traffic along a carriageway and in particular, but not exclusively, to determining the queue status thereof.

Traditionally, carriageway-mounted loop detectors have been used for many years for traffic detection. These detectors operate in a manner similar to a metal detector, with some turns of wire buried in the carriageway and usually wound in a square or rectangular form to form a sensing loop which is connected to a remote processor. The processor looks for an increase in frequency when a metallic object such as a car or other vehicle is fully or partially over the sensing loop. In many instances, the turns of wire which comprise the sensing loop are 2 metres long, in the direction of travel of the vehicle. The carriageway-mounted loop- based detectors are capable of providing vehicle count information by providing an output every time a vehicle passes over the sensing loop; in addition, if a vehicle is stationary over the sensing loop this can be detected and can be used to inform a traffic control system that a queue has formed. As vehicles are regularly passing and stopping over the loop, data is generated which is indicative of the amount of road which is occupied at a given time. The data generated for this occupancy is a function of the sensing loop dimensions in the direction of travel.

Although such sensing loops provide useful information to the traffic control system, they are costly and disruptive to maintain and install due to their being buried in the carriageway. Also the loop detectors have a finite lifespan with the operational life being a function of the quality of the road surface and its S S 5 * S * S S S * ** * : : * * S. *** S S usage.

Doppler effect detectors are used to monitor the velocity of a moving vehicle within the range of the microwave beam transmitted from the Doppler sensor. Doppler sensors work on the principal of emitting high frequency signals within a specified frequency range typically in the GHz region. A vehicle moving into or through the detection area reflects the signals back to the detector. The shift in frequency due to the Doppler effect is detected and used to determine the speed of the vehicle. Doppler detectors are essentially dynamic and react to motion only.

We have developed a system which overcomes this limitation of Doppler effect detectors and provides a signal indicative of the traffic condition along a carriageway which can be supplied to a traffic control system instead of the signal normally provided by the carriageway mounted loop detectors.

Accordingly, in one aspect, this invention provides a method of determining the condition of traffic of vehicles along a carriageway, which comprises: deriving from a Doppler effect detector directed towards said carriageway: (i) frequency data representative of the number of vehicles detected or speed detections in a selected time period, and (ii) rate data representative of the acceleration or deceleration of the detected vehicles or the rate of change of the value of speed detections, and using at least one of said frequency data and said rate data to categorise the traffic condition according to a plurality of predetermined conditions. S S *

* S S S S * S * * * * * * * * ... * : : : S S S ** * S S S * * S *S S S. S S

S

It will be appreciated that this information is derived from a Doppler Effect Detector which is mounted to face the carriageway and so can be installed, maintained etc. without requiring digging up of the carriageway.

In this specification, the term frequency data' should be interpreted broadly as meaning any form of data representative of a number of speed detections; it could simply be a count within a preset time period or it could be normalised to a frequency in terms of detections per minute. Likewise, the term rate data' is to be interpreted broadly as representing the rate of change of the value of the speed detections; this could be simply a qualitative indication of whether the detections are decreasing or increasing, or it may be a quantitative measure of the acceleration or deceleration.

Preferably, said frequency data and said rate data are used in conjunction with a state engine to determine the current traffic condition. The state engine may comprise discrete finite states corresponding to one or more of the following conditions: * Empty * Free Flow * Traffic Slowing * Congested * Queue Condition * Traffic Speeding Up Respective threshold values relative to the frequency data and/or the rate data may be associated with transitions from one state to another.

Preferably, the threshold values are programmable, and may S. *** *** . . . * * * * * * ... * : : : * * S *S S * * * S * ** S ** *** *.* conveniently be adaptively adjusted in accordance with historical profile data.

The data derived from the Doppler effect detector may be adjusted to correct for cosine effects due to an offset between the mounting position and the traffic movement.

In another aspect, this invention provides an apparatus for determining the traffic condition of traffic of vehicles along a carriageway, which comprises: a Doppler effect detector for being directed towards said carriageway in use, and processing means associated with said detector for determining: (i) frequency data representative of the number of vehicles detected or speed detections in a selected time period, and (ii) rate data representative of the acceleration or deceleration of the detected vehicles or the rate of change of the value of speed detections, and said processing means being further operable in use to categorise the traffic condition according to one of a plurality of conditions, based on said frequency data and said rate data.

Depending on the particular application, the processing to produce a frequency data and rate data and categorisation of the traffic condition may be performed in a single processor or it may be carried out at different, remote processors.

Whilst the invention has been described above, it extends to any inventive combination of features set out above or in the following description.

The invention may be performed in various ways, and an embodiment *. .*. *** * *. .S : : : .: : . : : * *. ..* *.* *.S thereof will now be described by way of example only, reference being made to the accompanying drawings in which: Figure 1 is a schematic view of a traffic monitoring scheme in accordance with this invention, and Figure 2 is a diagram of a state engine used in the processor of the embodiment of Figure 1.

It is widely known that Doppler radars can detect the movement of vehicles, and measure their speed, but that they provide no information when the potential targets are stationary, owing to the physics of the Doppler effect on which they operate.

In the embodiment described below, a Doppler radar is used for the detection of queues by the adoption of a novel processing approach. The embodiment operates on the basis that, in order for a queue to form, vehicles must have moved into the position they are currently in. In the embodiment, a Doppler radar measures the speed of the vehicles on the area of road of interest and when the speed falls below that of a predetermined level, say 10mph, then a permanent queue type output is generated mimicking that of a ioop detector as if a car was stationary over the loop. This may be used by a traffic control system in place of the output from a conventional carriageway-mounted loop.

The vehicles may eventually come to a stop, in which case no information will be received by the Doppler radar as it responds to movement only, but the queue output will still be present. In order to filter false alarms that may be generated by the occasional slow moving vehicle such as a bicycle, milk floats etc., the system may operate to acquire several consecutive speed readings * * S S S S. * * * * * * *.. * * : : * * ** a * * * * * * * * : S. * S below a preset queue threshold.

The queue threshold can be adaptively adjusted depending on the speed or signal data being processed at a given time which may optimise performance.

When the queue condition starts to clear by the vehicles beginning to move again, then information once more is available to the Doppler radar and the radar returns to measuring the speed of the vehicles. The queue condition output by the Doppler radar is cleared when a measured queuefree level is reached which normally is in excess of the original queue condition (i.e. vehicle speed of greater than 10mph). As with the queue onset level, it may be that several consecutive speed readings in excess of the threshold are required, or the threshold value for the queue-free speed could be adaptively adjusted as before.

Referring now specifically to Figure 1, in this embodiment, a Doppler effect detector 10 is positioned on a post to one side of the carriageway and supplies signals to a processor 14. The detector 10 is directed towards the oncoming traffic and emits a radar signal in the microwave waveband. The signal is reflected back to the detector modulated with a phase shift which depends on the speed of the object causing the reflection. The processor applies suitable filtering to reduce spurious readings due to clutter and noise and processes the signal to provide a speed detection for each reflecting object

currently in the field of view.

From the successive speed detection signals calculated by the detector, two parameters are determined: (i) the number (F) of speed detections obtained in a given time period IS. .*5 **. a * * : : :* a. 5 5 which can be normalised to give a frequency of detections per minute, and (ii) the rate of change (R) of the value of the detections obtained, which is analogous to the acceleration or deceleration of traffic.

The processor 14 also implements a state engine (see Figure 2) made up of a series of discrete finite states Empty', Free Flow', traffic Slowing', Congested', Queue Condition' and Traffic Speeding Up'. The transitions between these states are made dependent on the changes in value of the parameters F and R relative to preset threshold limits LI... L8, thus, for example, on start up or following a sustained period of no detections, the processor may assume an Empty' state. On detecting an increase in the number of speed detections, F, beyond the first threshold LI, a state of Free Flow' is assumed.

From here, if the frequency drops below another threshold, L2, the Empty state is again assumed. On the other hand, if, from the Free Flow state, the rate of change of value of the detections obtained falls below a further threshold L3, (L3 being negative and indicating significant decelerations of the vehicles within the field of view) a Traffic Slowing' state is assumed). In this state, if traffic in the field of view starts to slow further, to the extent that R is less than the threshold L4, then a Congested' state is assumed.

From the Congested' state, if the frequency falls below a threshold L5, then a Queue Condition' is assumed. If the frequency F then picks up and exceeds a further threshold L6, then the Congested' state is assumed again. If the rate R then passes a threshold L7 then a Traffic Speeding Up' state is assumed and if the rate R then increases beyond a further threshold L8 the *. *S. **. . * **.

* * S * * * * * * * S. S S *5 S * * * *.S *.S *** *.* *.* Free Flow' state is again assumed.

* * * S * * S S S S S a * * * * S S * * * *** * S * S a. S * ** * S S S S S S S * * * S ** * *5 * S

Claims

1. A method of determining the condition of traffic of vehicles along a carriageway, which comprises: deriving from a Doppler effect detector directed towards said carriageway: (i) frequency data representative of the number of vehicles detected or speed detections in a selected time period, and (ii) rate data representative of the acceleration or deceleration of the detected vehicles or the rate of change of the value of speed detections, and using at least one of said frequency data and said rate data to categorise the traffic condition according to a plurality of pre- determined conditions.

2. A method according to Claim 1, wherein said frequency data and said rate data are used in conjunction with a state engine to determine the current traffic condition.

3. A method according to Claim 2, wherein the state engine comprises discrete finite states corresponding to one or more of the following conditions: * Empty * Free Flow * Traffic Slowing * Congested * Queue Condition * Traffic Speeding Up * S S * a a S * 5 * * * * 55 e:. *: : : S. S * S * * * I * S

4. A method according to Claim 3, wherein respective threshold values relative to the frequency data and/or the rate data are associated with transitions from one state to another.

5. A method according to Claim 4, wherein said threshold values are programmable.

6. A method according to Claim 5, wherein said threshold values are adaptively adjusted in accordance with historical profile data.

7. A method according to any of the preceding claims, wherein the data derived from the Doppler effect detector is adjusted to correct for cosine effects due to an offset between the mounting position and the traffic movement.

8. Apparatus for determining the traffic condition of traffic of vehicles along a carriageway, which comprises: a Doppler effect detector for being directed towards said carriageway in use, and processing means associated with said detector for determining: (I) frequency data representative of the number of vehicles detected or speed detections in a selected time period, and (ii) rate data representative of the acceleration or deceleration of the detected vehicles or the rate of change of the value of speed detections, and said processing means being further operable in use to categorise the traffic condition according to one of a plurality of conditions, based on said frequency data and said rate data.

9. Apparatus according to Claim 8, wherein said frequency data * processing means and said rate data processing means comprise a single processor.

10. Apparatus according to Claim 9, wherein said frequency data processing means and said rate data processing means comprises separate processors.

11. A method substantially as hereinbefore described with reference to either of the accompanying drawings.

12. A method substantially as hereinbefore described with reference to and as illustrated in either of the accompanying drawings. *:.