GB2165982A - Traffic light control system - Google Patents

Abstract

A traffic light control system has two signal heads with associated detectors to sense approaching vehicles. A common control unit (1) has a setting-knob (2) which is turned to a distance mark on a scale (4) corresponding to the actual distance between signal heads. This sets a mandatory all-red period and a maximum green period for each head. Traffic flow may extend a minimum green period up to this limit. If this maximum is obtained a number of times in succession, it is extended to a new maximum, and similarly beyond that to a further maximum. When these new limits are not met, there is reversion to the previous maximum. The all-red period is not affected. <IMAGE>

Description

SPECIFICATION Improvements relating to traffic light control systems This invention relates to traffic light control systems. It is primarily concerned with those set up temporarily at road works or other hazards.

It will be assumed that there is just one signal head for each of two opposite directions, although in practice there may well be two, one on the near side and another on the offside of the road. It will also be accepted that there are microwave detectors capable of distinguishing a vehicle approaching each signal head and of supplying a signal to a central control.

When setting up such lights, account has to be taken of the distance between signal heads and the likely time that traffic will take to pass between them. A minimum green time of reasonable length has to be offered, and when that is finished at one end, there must be an allred safety interval before the other signal head turns green. Reference to amber in the specification will be omitted for simplicity.

Current practice involves the setting of individual timing switches for the lights, and the timings have to be calculated or derived from a chart relating distance between signal heads to acceptable periods. This is quite a complicated and lengthy procedure. Also, although there is usually provision for extending minimum green times up to a maximum according to traffic flow demands, the maximum is set by an operator and not adjusted throughout the day. This quite often proves inadequate to maintain a reasonable flow.

It is the aim of this invention to increase the flexibility of such a system and to make setting up easier.

According to one aspect of the invention there is provided a traffic light control system having two signal heads for opposed traffic flows, and control means whereby a green and red timing pattern is imposed on the heads, wherein said control means has a single selector member which, when adjusted on a scale corresponding to distances between the signal heads, sets a mandatory all-red period commensurate with that distance.

Generally, there will be means for detecting vehicles approaching each signal head and for generating variations in said timing pattern in accordance with traffic flow.

The selector member may also set a maximum green time, and there may further be means for assessing traffic flow in each direction from the output of the vehicle detecting means, and for adjusting the maximum green period according to the traffic flow.

According to another aspect of the present invention, there is provided a traffic light control system having two signal heads, control means whereby a red and green timing pattern including a mandatory all-red period is imposed on the heads, means for detecting vehicles approaching each signal head, and means for assessing traffic flow in each direction from the outputs of the vehicle detecting means and for adjusting the maximum green times according to the traffic flow.

In these systems the attainment of a predetermined successive number of maximum green times for either signal head can trigger an increase in the maximum green time for that head.

Subsequent failure of traffic to generate the new maximum green times will cause reversion to the previous maximum. This principle may be applied further so that there can be another extension to a second maximum.

These variations will preferably not affect the mandatory all-red period. Also, variations in the maximum green time generated by traffic flow may be proportionately less the greater the distance between the signal heads.

Generally, the system will be set for vehicle actuation of the detectors, but there will preferably also be an alternative manual control which can be operated if desired.

For a better understanding of the invention, one embodiment will now be described, by way of example, with reference to the accompanying drawings, in which: Figure 1 is a face view of the panel of a controller for a traffic light system, and Figure 2 is a diagram illustrating various timing relationships.

The controller has a front panel 1 on which there is a single knob 2 with a pointer 3 that cooperates with an arcuate scale showing distance ranges. The knob 2 is indexable to point to any one of those ranges, which correspond to the spacing of opposed signal heads. To either side of the scale 4 there are signal head repeaters 5 and 6 with lights that are energised in correspondence with those on the actual head.

The system can be vehicle actuated, and for this a membrane switch 7 is pressed. Alternatively, it can be manually operated by first pressing a membrane switch 8. Manual operation entails the selective pressing of further membrane switches 9, 10, and 11. Pressing the switch 9 produces the same effect as a vehicle approaching the signal head associated with the repeater 5, and inserts a demand for green at that signal head into the system. Likewise, the switch 10 is used to demand green at the signal head associated with repeater 6. The central switch 11 turns both heads red provided the minimum green period has expired.

Above the scale 4 there is a warning light 12 to indicate if the greens are in conflict, and another light 13 which flashes to indicate whenever the system is in a limit condition such as operating on maximum green. Also shown on the panel 1 is a main on/off switch 14 and fuses 15.

It will be appreciated that once the signal heads have been set up and connected, and their distance apart ascertained, all the operator has to do is turn the knob 2 to the correct range and switch on. This selects the all-red period and the maximum green time for both signal heads.

Normally, vehicle actuation will be the required mode, and this is automatically selected on switch-on. It will be understood that this maximum green time is not necessarily attained every time; traffic demands have to generate extensions to achieve that maximum and there may not always be enough to do so.

The operation of the system will be described for this vehicle actuation mode.

The closure of the switch 7 causes a simulated vehicle signal oF demand to be applied first to one and then to the other signal head, to turn the heads green alternately and separately. This will get any stationary traffic (which would not register at the detectors) moving.

If no vehicles are thereafter present, both signal heads A and B (not shown, but distinguished thus for reference to Fig. 2) will be at red and remain so indefinitely. There can, however, be a timing circuit which would turn the heads green alternately at set intervals for brief periods to insure against detector failure.

If a vehicle approaches signal head A, say, when both heads have been at red for more than the all-red period set by the knob 2, then its movement is detected and generates a demand for that signal head A to turn green. It does so either after the vehicle has stopped in front of it, or while it is still approaching. The signal head will be guaranteed a minimum green time, say 12 seconds, but the vehicle will cause it to be extended. After moving off from its temporary stop the vehicle will still be in the zone of detection and will not move clear of it perhaps for several seconds. Likewise, a continuously moving vehicle may remain within that zone for a short period after the signal head has turned green. The departure from the zone in itself generates a signal, and from this time the green period is extended, by a period of 15 seconds say.This is the maximum extension generated by that one vehicle, but it can be further extended or foreshortened, as described below.

A following vehicle detected whle the signal head is still green will prolong the green period. It will then end 15 seconds after the second vehicle moves out of the detection zone. This situation is illustrated diagrammatically in Fig. 2 (i) where the broken lines represent a red signal, the full lines a green signal, an arrow pointing towards the line a vehicle detection, and an arrow pointing away from the line a vehicle leaving the detection zone. In this example no vehicle approaches signal head B. Further vehicles approaching signal head A while that is green will extend the green period yet more, but there will be a finite limit (the maximum green time) imposed by the setting of the knob 2.

Fig. 2 (ii) illustrates another possibility, where a vehicle approaching signal head B is detected before the 15 second extension period at signal head A has commenced. The effect of that is to insert a demand which shortens that extension period, for example to 7 seconds, although it may be as brief as 2 seconds. There is then a mandatory all-red period, followed by the signal head B turning green.

In the situation illustrated by Fig. 2 (iii), a vehicle approaches signal head B while signal head A is at green and the 15 second extension period is running, although for less than the extension period (7 seconds) of situation (ii). The result of that demand is the termination of the extension period at its foreshortened time of 7 seconds. There is then a mandatory all-red period, after which the signal head B turns green.

In the situation of Fig. 2 (iv), a vehicle approaching signal head B is detected while signal head A is at green and the 15 second extension period has been running for longer than the foreshortened extension period of situations (ii) and (iii). The effect of that is to terminate the extension period immediately so that both heads go red, followed by the signal head B turning green.

In Fig. 2(v), signal head A has turned from green to red and the mandatory all-red period has expired, although there is still an all-red condition, before a vehicle is detected at signal head B.

that is free to turn green immediately. The same would apply if B has been green instead of A before the all-red period. In other words, it is 'first come, first served' in the absence of timing control interference after the all-red period.

In Fig. 2 (vi), signal head A has turned from green to red and the all-red period has been running for more than two seconds when a vehicle is detected at signal head B. The manadatory all-red period times out, and then the signal head turns green. A following demand at signal head A during the all-red period or later will be registered and that signal head will turn green subsequently.

In Fig. 2(vii), signal head A has turned from green to red and the all-red period has just commenced having run less than two seconds, when vehicles are detected at both signal heads.

It does not matter in which order; after the mandatory all-red period signal head B turns green, and signal head A will follow to green later.

In Fig. 2 (viii), signal head A has turned from green to red and the all-red period has been running for more than two seconds when a vehicle is detected at signal head A, followed by one at signal head B. The former takes precedence, and signal head A turns back to green at the end of the mandatory all-red period. The other demand is registered, however, and signal head B will turn green subsequently.

In addition to these relationships, which are resolved and acted upon by a microprocessor in the control unit, there will also be the minimum green recall facility substantially as described in our copending Patent Application No. 8405037. That is a system which caters for the failure of a vehicle which has turned a signal head green to clear the detection zone during the minimum green period. Should there be no green period extension triggered by departure from such zone a simulated demand is inserted and that signal head is returned to green as soon as is compatible with any other demands. That recall may be repeated if the first one again produces no extension.

Reference was made above to the green period being extended to a maximum. When there is appreciable traffic flow, it is accepted practice to allow the minimum green period to extend so that more than just two or three cars can move through. Such extensions are automatically generated by the vehicle detectors sensing the stream of traffic approaching and triggering the controller to grant extensions. But a limit has to be set in order to allow traffic from the opposite direction to move, and as mentioned previously this is programmed in by the knob 2.

However, while one maximum may be satisfactory for moderate traffic flow, it may be insufficient for heavy traffic flow, and thus cause tailback at the lights. It is therefore desirable to have a facility for increasing the maximum green time.

The maximum green time, as mentioned above, is controllable by the knob 2, but adjusting that will also affect the mandatory all-red period, and while it may be advantageous to lengthen the maximum green time, extending the all-red period in correspondence will normally be unnecessary and slow up traffic flow. Also it requires an operator to stand by the knob 2 and adjust it as he thinks fit.

The proposed system is capable of extending the maximum green time automatically, and making it revert to its set value when traffic flow eases, this without affecting the all-red period.

The controller is programmed to register when a number (say two) of successive maximum green times have been attained. This signifies that traffic is heavy. To make allowance for this, the maximum green time for that signal head is increased by say 10 seconds. This increased maximum green time is then allowed for another two (say) green periods of the signal head. If the new increased maximum time is still being used, a further extension of 10 seconds may be granted for the next two green periods. However, if traffic falls off, then that signal head will revert to the previous, lower maximum green period say after two cycles. The all-red period stays constant throughout.

This variability is particularly desirable for the lower settings of the knob 2. However, where the signal heads are very far apart, the maximum green period is generally quite substantial, and this facility for its increment may not be made available. The following table illustrates typical allred periods and maximum green times for various distances between signal heads, and the part relating to green times illustrates progressive increments to and decrements from an upper limit for the lower ranges and the levelling out for the higher ranges.

SABLE. I

DISTANCE BET WEEN ALL RED TRAFFIC SIGNALS PERIOD (SECONDS) VARIABLE MAXIMUM GREEN (SECONDS) (@ETRES) PRE@@T INTER MEDIATE UPPER INTERMEDIATE PRESS@ VALUE ST@ @ u @ LIMIT STEP @OWN VALUE (2 MAX CH@@@@@) (2 reduced changes) 0 - 15 2 15 25 35 25 15 15-30 2 20 30 40 30 30-45 5 25 35 45 35 25 45-75 10 30 40 50 40 30 75-105 15 35 45 55 45 35 105-135 20 40 50 60 50 40 135-165 25 45 55 60 50 45 165-195 30 50 60 60 50 50 195-250 40 55 60 60 55 55 Fixed time operation could be achieved either by disconnecting both vehicle microwave detectors or by providing a fixed time switch to cut out the detectors.

The system can also incorporate the minimum green recall feature with a timed extension as described in Application No. 8405037 and the corresponding U.S. Application.

Claims (10)

1. A traffic light control system having two signal heads for opposed traffic flows, and control means whereby a green and red timing pattern is imposed on the heads, wherein said control means has a single selector member which, when adjusted on a scale corresponding to distances between the signal heads, sets a mandatory all-red period commensurate with that distance.

2. A system as claimed in claim 1, including means for detecting vehicles approaching each signal head and for generating variations in said timing pattern in accordance with traffic demands.

3. A system as claimed in claim 2, wherein said selector member also sets a maximum green time.

4. A system as claimed in claim 3, including means for assessing traffic flow in each direction from the output of the vehicle detecting means, and for adjusting the maximum green periods according to the traffic flow.

5. A traffic light control system having two signal heads, control means whereby a red and green timing pattern including a mandatory all-red period is imposed on the heads, means for detecting vehicles approaching each signal head, and means for assessing traffic flow in each direction from the outputs of the vehicle detecting means and for adjusting the maximum green times according to the traffic flow.

6. A system as claimed in claim 4 or 5, wherein the attainment of a predetermined successive number of maximum green times for either signal head triggers an increase of the maximum green time for that head, and wherein subsequent failure of traffic to generate the new maximum green time causes reversion to the previous maximum.

7. A system as claimed in claim 6, wherein a new maximum green time, when attained another predetermined number of times in succession, triggers a further increase in the maximum green time.

8. A system as claimed in any one of claims 4 to 7, wherein variations in the maximum green time generated by traffic flow do not affect the mandatory all-red period.

9. A system as claimed in any one of claims 4 to 7, wherein variations in the maximum green time generated by traffic flow are proportionately less the greater the distance between signal heads.

10. A traffic light control system substantially as hereinbefore described with reference to the accompanying drawings.