GB2298945A - Traffic light control systems - Google Patents

Abstract

The system has moving vehicle detectors which govern the changeovers between red and green in accordance with traffic flow, giving a minimum green period to a detected vehicle, which may have to be delayed while an opposed green period expires and an inter-green time elapses. A leading vehicle may fail to respond promptly to a facing green and the minimum green time may then expire with no traffic flow. But vehicles moving up briefly behind the leading one will signal that a queue has formed and the minimum green time will be extended to give the queue a further chance of moving. No detection of movement during a green period will cause opposed lights, if they have been primed by an approaching vehicle, to turn green for a period before another green period is automatically granted to the original lights.

Description

n ImProvements relatina to Traffic Light Control Systems" This invention relates to traffic light control systems. It is applicable both to permanently installed lights, and to temporary ones set up at road works, for example.

In this specification a complete installation of lights will be referred to as a set of lights, while for each approach it will be assumed that there is a "unit" of lights. In practice, there may be just one signal head, or two, for example one on the near side and one on the off side of the road. Also, there will usually be an amber light to show during changeovers between red and green, but reference to this will be omitted for simplicity.

Many sets of traffic lights use microwave detectors to sense the approach of vehicles. If there is just one vehicle, an electronic control system will set the appropriate light to green and the vehicle can move through.

It may have to stop while its lights turn from red to green.

If there are two vehicles approaching from different directions, then the one sensed first will be given precedence, while the other will be held by a red light. However, once the first vehicle has cleared, it is within the capacity of the control system to remember the detection of the other one if it has by then stopped, and to turn its lights green after an appropriate interval.

The same general principles apply for more than two vehicles approaching a three or four-way system of lights from several directions.

It is usual for such a system to grant a minimum green time for a vehicle to get under way and start to move through the lights. Commonly, this is 12 seconds, but there may be provision for extending it, as described in GB-B2135803. In that, a minimum green period is initiated by a first detection, and then a second detection, within that period, is used to check consequential traffic flow. If it does not materialise, that period is extended. The controller deduces from the first detection that there is a vehicle there waiting to pass through, but has somehow been temporarily thwarted. It is therefore given more time to get moving.

Another proposed measure is described in our Patent No. 2273598B where lack of a second detection,is used to extend the inter-green time, thus providing some insurance against a vehicle getting started just after the green phase has finished and jumping the lights.

A further problem is addressed by our co-pending Application No. 9325811.9. This is where the leading vehicle may be stuck, and the ones behind, seeing the lights turn green, may shunt up and then have to stop. This will give a second detection and suggest falsely to the control means that there is normal traffic flow and that there is no need to extend or recall the period once it has finished.

The second detection, if lasting for less than a predetermined sub-period, can be arranged to extend the green period.

But there is also the situation where a vehicle has failed or is unable to move after generating a green period at one unit and another vehicle, approaching from the opposite direction, inserts a demand for green at the other unit. The driver of the second vehicle may become aware that the first vehicle is stranded, and may therefore be tempted to jump the lights. At the least, it is unreasonable to keep him waiting.

It is the aim of this invention to reduce the likelihood of traffic lights being disregarded, while giving further assistance to free traffic flow.

According to the present invention there is provided a traffic light control system comprising a moving vehicle detector for each unit of a set of lights and control means responsive to the detection of a , moving vehicle to turn the lights of the unit associated with the detected vehicle green after ensuring that the lights of the other units are compatible, wherein a green period is initiated by a first detection, and wherein if there is no further detection by that unit but a detection by another unit during the green period, that other unit is turned temporarily green to satisfy that demand and then the first unit is returned to green.

Generally the other unit will be granted a normal minimum green period (twelve seconds in current practice) to clear its approaching traffic, and then the first unit will go green again to clear the vehicle(s) presumed to have stalled or otherwise been impeded after the original detection. This temporary green period of said other unit may be extendible as a result of nil or minimal traffic flow in like manner to the extension of the original green period as described in GB-B-2135803 or in Application 9325811.9, referred to above.

Should there be no opposing demand (that is no detection by another unit during the original green period), that period may be extended up to a limit. Generally, the original green period will be the normal minimum twelve seconds and the extension could be, for example, one or two minimum green periods, making a total green period of 24 or 36 seconds. If there is still no detected movement, then the unit is returned to red.

Should there be an opposing demand at any time while the first unit is at green, then the green phase of the first unit may be terminated either at the end of whatever minimum green period (first, second or third) is running at the time or later. For example, an opposing demand during the first minimum green period may be held while that first period is extended by a second one. Then, if there is still no traffic movement, after a suitable inter-green time the opposing unit is turned green to clear its traffic. Finally, in this cycle, the first unit is recalled to green for one, two or more minimum green periods to give its traffic a chance to move again.

Claims (6)

Claims

1. A traffic light control system comprising a moving vehicle detector for each unit of a set of lights and control means responsive to the detection of a moving vehicle to turn the lights of the unit associated with the detected vehicle green after ensuring that the lights of the other units are compatible, wherein a green period is initiated by a first detection, and wherein if there is no further detection by that unit but a detection by another unit during the green period, that other unit is turned temporarily green to satisfy that demand and then the first unit is returned to green.

2. A traffic light control system as claimed in Claim 1, wherein said other unit is granted a normal minimum green period when turned temporarily green.

3. A traffic light control system as claimed in Claim 1 or 2, wherein the temporary green period of said other unit is extendible as a result of nil or minimal traffic flow in like manner to any extension that can be made to the original green period.

4. A traffic light control system as claimed in Claim 1, 2 or 3, wherein the green period of the first unit comprises a minimum green period and extensions thereof, and wherein, when a detection is made by another unit during the minimum green period or an extension thereof, the temporary green period of said unit commences after expiry of an extension beyond the minimum green period or after expiry of an extension beyond the extension in which that detection occurred.

5. A traffic light control system as claimed in any preceding Claim, wherein, if there is no detection by another unit during the original green period, that period is extended.

6. A traffic light control system as claimed in Claim 5, wherein the extension of the original green period is limited.