Traffic light system

Abstract

A system of traffic lights 102,104 is controlled 150 so that their green time is a minimum green time plus a green time quota, the latter distributed across the lights 102,104 depending on a level of

Description

This invention relates to traffic light systems and to a method of operating a system of traffic lights. It is especially but not exclusively applicable to temporary traffic light installations. Traffic signals are often required to be deployed on a temporary basis in order to facilitate highway maintenance or the connection or servicing of utilities, respectively known as road works and street works. Tn some cases, it is a set of permanent traffic signals requiring maintenance that create the need for temporary signalling. Often works are required to take place in locations where their presence has the potential to significantly impact journey times and road user experience on a specific route. Highway authorities and traffic management companies work to minimise delays to traffic however this job is made more difficult by a number of factors. Works often change the road layout / capacity as compared with the normal road layout. This means that vehicle flows can be significantly reduced. It also means that any existing knowledge of appropriate signalling timings likely is no longer applicable. The short-term nature of the works means that resources required to fully model the impact of the temporary signalling are not cost effective and would be in any case difficult to resource in a timely manner. Emergency works are sometimes required where there is no notice prior to the signals being required. These factors create the need for a system of temporary signalling which can be deployed with minimal prior modelling or preparation but which is able to respond to traffic conditions intelligently. Such systems exist already (for example the AutoGreen ® system). A key feature of such systems is their ability to van the amount of green time which is available to each phase / stage of the signals. Longer green times are more efficient due to a reduction in the proportion of time lost to the intergreen period. The use of longer green times can be preferable when traffic flows are high provided that traffic does not become congested as a result of their use. This approach works well for simple 2-way sites (i.e., a site with two traffic phases / stages for which the system alternates showing green to one then the other). A problem arises however at more complex junction. If we take a 4-way site (e.g., a crossroads) where work in being performed in the carriageway such that only one lane is available. This type of ‘shuttle’ working significantly reduces vehicle throughput. Increasing green time on the four phases is possible however it quickly introduces a new problem, namely an excessive ‘cycle time’ and a low overall vehicle throughput. The cycle time for a set of traffic signals is the time required to service (show green to) each traffic phase and return back to the first phase in the sequence. Table 1 below shows the cycle time and maximum throughput of vehicles on each phase that is achieved for a range of schemes with differing numbers of phases. Cases for a range of different 5 maximum green times are shown where it assumed that each light is illuminated for the maximum time. Note that vehicle throughput values are based on empirical observation collected across a number of real sites. An additional amount of red time of 10s between each phase is assumed. Number of Phases Max Allowed actual Green Time for each phase (Seconds) Cycle Time (Seconds) Vehicle Throughput Per Phase (vehicles per hour) 2 25 80 432 2 45 120 624 2 70 170 736 4 25 160 216 4 45 240 312 4 70 340 368 6 25 240 144 6 45 360 208 6 70 510 245 10 Table 1 Table 1 above shows clearly that at sites with more than two phases (i c.. ‘multiphase’ sites), the cycle time for the signals rapidly exceeds an appropriate level. Note that permanent traffic signals recommend that cycle times should be maintained below 120 seconds at all times. Cycle times that are significantly higher than this such as 340 seconds = 5 minutes 40 seconds are generally considered to be unacceptable in most situations. Research has shown that faced with waiting for these lengths of time, many drivers will jump a red signal. Equally, holding traffic at a phase for this period of time in itself can cause gridlock. At present the applicant is not aware or any solution to this problem that achieves all of the desired outcomes. Temporary signals in shuttle works involving more than 2 phases simply cannot move a volume of vehicles per hour as would be required on a key arterial route. Queuing is therefore inevitable. The challenge becomes one of minimising and also managing the creation of queues. The invention described here targets both of these aspirations. Systems have been developed which automatically vary green time on portable and traffic signals. One example is known to the skilled person by the name Vehicle Actuation: This is an industry standard method of varying green times, widely used as it is built into the TOPAS specifications for equipment. Vehicle Activation (VA) has been around since the 1970s in its current form and before this in other forms. VA is limited to a simple automatic extension of green times to a value between a pre-defined minimum and maximum green time setting. On each cycle, a phase will show green for the minimum green time period. The phase will then remain at green provided that the detection sy stem (usually radar) observes continued demand for the phase (i.e., detects the presence of approaching vehicles). This continues up to a maximum green time where the phase then returns to red, and another phase is serviced. There are some variations and nuances to this behaviour but the above represents the core functionality that would be experienced during the busy periods we are considering here. Another prior art scheme is that sold and marketed under the name AutoGreen ® by the applicant. This system automatically varies actual green times (and other signalling behaviours) in response to traffic levels. Green times are determined across multiple cycles, not just a single cycle as is the case with VA. It is limited to 2-way operation only. The system increases or decreases the maximum green time by 5s at the end of each cycle. It also varies behaviours of the signals according to how busy the phase is. This is achieved through the use of Scenarios. There are Scenarios for light traffic right up to heavy traffic and then congested traffic. A wide range of functionality in the signals varies according to the scenario currently being applied. Whilst VA provides excellent control for may installations, especially those used in low traffic areas, the system can lead to cycle times that become excessive at peaks times which is not ideal and can lead to impatient drivers jumping the red phase of the lights if they think there is a fault that might mean the green not appearing. According to a first aspect the inventions provide a traffic light system comprising at least two traffic lights which are operated in a repeated sequence of cycles, each traffic light comprising a green light which indicates that vehicles may pass the traffic light when the green light is illuminated, at least one control unit that is arranged to generate a respective control signal for each traffic light that determines a requested actual amount time for which the green light is illuminated within the cycle, each traffic light being responsive to the respective control signal to illuminate the green light of that traffic light for the requested amount of time within a cycle, and a memory which stores data for use by the control unit to set the duration of the requested illumination of the green lights, the stored data comprising: a minimum green time value indicative of the minimum duration for illumination of a green light of each traffic light during a cycle; and a green quota reserve value indicative of an overall amount of additional time for illumination of the green lights over the minimum green time value during a cycle that can be allocated across the traffic lights, and in which the control unit is arranged to set the actual demanded green light time by allocating time from green quota reserve across the traffic lights as a function of a level of traffic demand at each traffic light such that the actual minimum time value for each light is met or exceeded and that the total additional time allocated across all of the green lights of tine system does not exceed the value of the green quota reserve. The memory' may store a value representing the green quota reserve, or a set of values which when totalled by the control unit define the overall value of the green quota reserve. This value may be fixed by an operator during installation or commissioning of the system. The memory may store a maximum green value for the green light of each of the traffic lights which represents the maximum duration of illumination that the control unit must not exceed when allocating time from the green quota reserve. Tire control unit may only allocate time from the quota that will take the total illumination time for a green light up to the maximum illumination duration value such that any demand exceeding that is not met. The maximum time may be the same for each traffic light or at least traffic light may be given a different maximum time to the others. They may all be set individual maximum time values. The sum of the maximum times equals the maximum cycle time for the traffic lights. The mcmon may store a unique minimum green time duration for each traffic light, or a single minimum duration that is used for all traffic lights in the system. It may store a single default value and for each traffic light a variance value which can be combined with the default value to determine the minimum value for each traffic light. When the actual green illumination time for all traffic lights is set at the minimum, for instance when there is no traffic demand, this defines the shortest cycle time for the traffic lights. The system may assign a priority to each of the traffic lights such that the assignment of time from the green quota reserve to those with a higher priority differs from those with a lower priority. The control unit may take account of the priority or values set as a function of the priority when allocating quota reserve time. The memory may store a look up table of values that are associated with a priority and the control unit may be configured to assign values to each traffic light according to a user defined priority, these values being processed by the control unit when determining the actual green light illumination time. The control unit therefore does not directly access the priority but does act on values that are set as a function of the priority. The control unit may assign a green quota value to each traffic light representing the maximum amount of time that is guaranteed to be available to be assigned to that traffic light from the green quota reserve, should that time be required. This is an amount of time that is ring fenced for that traffic light and which the control unit may always be free to assign to that traffic light during a cycle. The control unit may allocate more time over this maximum green quota if there is time remaining in the green quota reserve and if it is required to meet a demanded time that exceed the maximum green quota up to the maximum illumination green for a traffic light. The maximum green quota value may be stored in the memory for access by the control unit. A higher priority traffic light may be assigned a higher green quota time than a lower priority traffic light. The ring-fenced green quota time is always available for allocation to that traffic light when calculating the actual illumination time in addition to any pre-set minimum green light illumination time for that traffic light. Ilie values of the minimum quota time value for each traffic light may be stored in the memory for processing by the control unit. Where each light has already been allocated additional quota time the control unit may only assign additional time to a green light time of a traffic light above the maximum green quota if there is unused quota time that has not been assigned to the other traffic lights. The control unit may make all decisions about the allocation of quota reserve time and setting the actual green illumination time for each traffic light at the end of each complete cycle (and not at any other time) or at the start of each cycle (and at no other time). At the end of a cycle or the start, when even traffic light has completed one cycle of transitioned from green to the next red phase via typically an amber light), the control unit may increase or reduce the value of the actual green illumination time for each phase. The control unit may reduce the actual maximum green time for any phase which was not experiencing a traffic demand up to the point of its maximum green time value during the immediately preceding cycle. So if during the last cycle, the traffic demand ceases after a set number second but the actual maximum green value was set to a longer time period, then the control unit may reduce the actual maximum green value for use on the next cycle by an amount. This may be a relatively small fixed amount of say 1 to 10 seconds. The control unit will not reduce the actual maximum green time for a phase below it’s quota value. This means that the quota is always available for a phase, even if it didn’t use all of it on any of the previous cycles. Equally, the phase will not be held at green for any longer than the min green value if there is no traffic. When setting a green light illumination time for one traffic light of the system the control unit may process during a cycle of the traffic lights one or more of the following: -the traffic level of demand signal associated with that traffic light; -a minimum green light illumination time assigned to the traffic light; -a maximum green light illumination time assigned to the traffic light; - the amount of quota time assigned to each of the other lights at tire time the calculation is being made; -the allocated minimum amount of quota time that each of the other traffic light is entitled to take; and - the actual green light illumination time allocated to the traffic light on the last cycle. The traffic light system may include a sensor apparatus associated with each traffic light that detects the presence of vehicles at the approach to the associated traffic light and outputs a traffic level of demand signal to the control unit that is used when allocating quota time. This signal may be a binary signal with a first value indicating that there is a demand for a vehicle to cross and second value indicating that there is no demand for a vehicle to cross. The value of this signal may be set by tire sensor apparatus in real time. This may output a binary signal with one value indicating a vehicle is present and the second indicating that no vehicle is present. It may also output a detailed stream of digital information about approaching vehicles such as position and speed, for interpretation by the control unit which then determines the level of traffic demand on each approach. The control unit may process this information to determine a level of demand signal. In a preferred arrangement the sensor apparatus may comprise a radar (or other) sensor that detects the presence of vehicles at or approaching one of the traffic lights. It may comprise multiple radar (or other) sensors with each one associated with a different respective traffic light. Where there are multiple sensors, each may generate a separate signal and the control unit may be arranged to generate the actual maximum green light illumination value The sensor apparatus may detect the speed of a vehicle in a predetermined zone in front of the signal and the duration may be varied by an amount that is as a function of the speed of any approaching vehicle detected by the radar. Where the speed of a vehicle falls below a threshold the duration may be the same as would be used if no vehicle is present, e.g., a shorter duration. If the speed is above the threshold the longer duration may be used The sensor apparatus may also or alternatively detect the position of the vehicle in the zone and the duration may be varied by an amount that is as a function of die position of any approaching vehicle detected by the radar. If the vehicle is a long way from the signal the shorter duration may be used, and if closer the longer duration may be used. The control unit may determine the level of traffic demand for green illumination time according to a known process for traffic lights such as that used in the prior art AutoGreen ® system developed by the applicant. The invention lies in determining the actual illumination times in response to the demands. Where the sensor apparatus indicates that there are no vehicles waiting to cross at the end of a minimum time of illumination of a green light the control unit of the system may reduce the period by assigning any or part of any assigned quota time back to the unused quota value for use by other traffic lights in the system. The sequence may comprise set of identical cycles that are repeated where each cycle comprises a pattern in which each green light of each traffic light is illuminated only once. Other patterns may be used where some lights are illuminated several times within each cycle when there are three or more traffic lights and the expected demand for each approach to the works in very different. More than one green light may be illuminated at a given time. For example, the traffic lights may be paired so that the duration and timing of illumination of both lights of a pair are the same. This will typically be a pair of lights directly opposite one another at a junction such as a crossroads when bi-directional traffic flow is permissible in the design of the works. In this case when the actual demand time for green of one of the pair is calculated by the control unit the value for the other traffic light of the pair will be set to match. This may be taken into consideration when deciding how much quota time can be allocated. The apparatus may include an interface through which an operator may generate or adjust the information stored in the memoiy. For example, the operator may generate or adjust the minimum green illumination times for the traffic lights, and may generate or adjust the green quota time and maximum green light illumination time for each traffic light. Where there is a priority for the traffic lights the operator may generate or adjust the priority and the system may determine a set of appropriate maximum and minimum green values from this. The user may also set a maximum cycle time and the system may scale the values in the memory to ensure this is not exceeded. This will typically be done when the traffic lights are installed and commissioned prior to use but then is likely to be adjusted (often remotely) after the traffic lights have been deployed in order to adjust the system based on observation of the impact of the signals on traffic, for example lengths of queues on each approach. The interface may comprise an input device such as a keyboard or touchpad or dials or sliders or push buttons or toggle switches or any combination of the above and comprise a display screen which displays the entered information or the stored information to the operator. Each traffic light of the traffic light system may comprise a portable or temporary traffic light that can be readily installed for a period of time and the removed for reuse at a different location. The traffic lights may include a base and a mast with the green light mounted high up on the mast. The skilled person will understand that in addition to a green light each traffic light will include a red light and typically also an amber light. The duration of the illumination of the red lights may be set by the control unit as a function of the road geometry and of any sensor inputs relating to vehicle movements. Generally the timing of the red lights will be set such that at any time in a cycle only one green light is illuminated and the red light of every other traffic light is illuminated, or where multiple lights are showing green multiple others are showing red. The system may include a single control unit that sets the green time for all the traffic light of the system. Alternatively, the system may include multiple control units. Each may have its own memory that stores the parameters used by the control unit or may access a shared memory where the parameters can be set and adjusted centrally. The control unit may be arranged to illuminate the lights by producing control signals indicative of the phase of the lights at any given time. The system may include, as a part of each traffic light, a drive circuit that supplies current to each of the lights of the traffic light dependent on the value of the control signal received from the control unit. The control unit may transmit control signals to each traffic light wirelessly, or to one traffic light via a wired connection and wirelessly to the other. These signals may instruct each traffic light which of the red, amber and green lights are to illuminated second by second, or at a higher or lower rate as appropriate for a given installation. The control unit may be an integral part of one of the traffic lights such that the traffic light acts as a master traffic light, control signals being sent to the other traffic light which functions as a slave traffic light. Each radar sensor may comprise an antenna, a signal generator that applies a waveform to the antenna and a signal processor that detects signals received by the antenna and from the transmitted signal and detected signal determines whether there is a vehicle within the detection range of the radar sensor. The radar sensor may transmit to the control unit a binary signal indicative of whether there is a moving vehicle or not, or a more detailed signal that may indicate one or more of: the number of moving vehicles detected, the distance from the traffic light to one or more detected vehicles, and the speed of one or more detected vehicles. The control unit may receive these signals from the radar sensors wirelessly. The traffic lights may comprise fixed traffic lights permanently installed at a junction or crossing. The traffic lights may comprise temporary lights, or portable lights. They may include a power source for supplying power to the lights and the control unit and the radar sensors so that the traffic lights can be operated without connection to a mains supply. The battery may be an integral part of one or more of the traffic lights or may be a stand-alone battery. As an alternative to a battery a generator may be provided that is petrol or diesel driven and produces an appropriate electrical output. According a second aspect the invention provides a method of controlling at least two traffic lights each traffic light comprising a green light, the method comprising operating each of the lights in sequence whereby the duration of each green light being illuminated within a cycle of a sequence is set by the following steps: (a) determining a demanded illumination duration for a green light of each of the traffic lights as a function of a level of traffic demand for each traffic light received from a sensor apparatus: (b) assigning a minimum green time value to each traffic light indicative of the minimum duration for illumination of a green light of a traffic light during a cycle; (c) setting a green quota reserve value indicative of an overall amount of additional time for illumination of the green lights over the minimum green time value during a cycle that can be allocated across the traffic lights, and (d) allocating time from the green quota reserve across the traffic lights as a function of the level of traffic demand for each traffic light to set an actual maximum green illumination time for each green light such that the minimum illumination time value for each light is met or exceeded and that the total additional time allocated to the traffic lights from the green quota reserve does not exceed the value of the green quota reserve. The steps (b) and (c) may be performed once during installation of tire traffic lights or periodically set or adjusted by an operator. The steps (a) and (d) may be performed during operation of the traffic lights, for example once each cycle at the end of a cycle or start of a cycle. The method may comprise stepping each value of actual green time up by a preset amount of time when the level of traffic demand is increased relative to that used when the actual green time was last set. Uris preset amount may be a value between 1 second and 10 seconds, preferably 5 seconds. Hie timing of the lights will therefore only van slowly regardless of how rapidly the demand changes. Similarly the method may comprise stepping each value of actual green time down by a preset amount of time when the level of traffic demand is decreased relative to that used when the actual green time was last set. This preset amount may be a value between 1 second and 10 seconds, preferably 5 seconds. The timing of the lights will therefore only vary slowly regardless of how rapidly the demand changes. The actual green time may be increased if the previous green time was insufficient to meet tine level of traffic demand, for instance if vehicles are still approaching the traffic light at the end of a green phase. The method may further comprise setting a priority for each traffic light and step (d) may allocate time from the green time reserve quota as a function of the assigned priorities. For example, the memory may store a maximum permissible value for the actual green light illumation time for each traffic light that is a function of the assigned priority and the control until may only assign quota time up to that maximum and only if there is time remaining in the overall quota to allow the extra time to be assigned to a traffic light. Because the minimum green light times are fixed and the overall quota time is fixed the maximum duration of a cycle is therefore limited preventing the excessive cycle times that may otherwise occur in prior art systems that determine the time for each traffic light individually with no regards to the duration set for the other lights in a system. The method may set a green quota time value for each traffic light representing anamount of time that light may always be able to take from the quota reserve when determining the actual green light illumination time. This is a ring fenced amount of time forthat light. Where each light takes all of it is ring fenced quota time the remaining quota time reserve may be equal to zero. Where one or more do not take all their ring fenced time this unused time will remain in the quota reserve to be assigned to other lights that wish to exceed their ring fenced quota. The method may take account of these allocated quota time values and the time assigned from the quota to all other traffic lights when determining how much quota time is to be assigned to a traffic light. Ideally the demanded time will always be met so that the actual time meets the ideally demanded time but the method will not do so if the total goes above a maximum allowed time for each traffic light or if there is insufficient unallocated quota time to be taken. During the cycle, each phase may effectively request what time it would ideally like to have during the next cycle. Just before the next cycle begins, the controller will perfonn the allocation of time to each of the phases based on what they have requested and how each phase is configured in terms of quota. Since that light will have been allowing more vehicles to pass during cycles it is to be expected that this will self-regulate with the allocated quota time moving around between traffic lights as demand is satisfied. The method may determine the actual green light illumination time for each light in the traffic light system in a fixed sequence, stepping through each light in turn and then repeating the sequence in time. Uris may be an arbitrary sequence set by the control unit on initial commissioning or may be predefined by a user of the system. There will now be described by way of example only one embodiment of the present invention with reference to and as illustrated in the accompanying drawings of which : Figure 1 is a schematic of an example of a traffic light system in accordance with the present invention installed at a four-way junction; Figure 2 is a more detailed view in plan of the junction from one side showing two of the traffic signals, the sensor apparatus and control unit; Figure 3 is a time chart showing how each green light in the exemplary system is illuminated only once during each cycle; Figure 4 is drawing illustrating the parameters used to determine the actual green light illumination time for Phase A of the system of Figure 1; and Figure 5 is a flow chart showing the steps carried out to determine the actual green light illumination time for a phase. As shown m Figure 1. an exemplary traffic light system 100 according to the present invention comprises four portable traffic lights A, B, C and D labelled as 102,104,106 and 108, each located at one approach to the works to control the flow of traffic safely through the works. For simplicity it is assumed that each of the traffic lights is identical. Figure 2 shows tire works from one approach where two traffic lights are visible one of which corresponds to tire approaching traffic from the left to control that traffic and the other facing another approach from the right (the two other traffic lights are obscured by these two lights in this simple example). Each light comprises a pole 110 which supports a light head 120, the light head comprising a housing for three lights - one green 130a, one amber 130b and one red 130c. Each of the three lights 130a-c in this example comprises an array of coloured light emitting diodes (LED’s). Each of the traffic lights 102,104, 106, 108 is connected through a network of cables or a wireless link to a control unit 150 which is responsible for generating control signals for the traffic lights. This unit 150 may be attached to or form an integral part of one of the traffic lights or may be located remotely from the traffic lights at the works site. The control unit 150 comprises a signal processor 151, a memory 152, and a set of program instructions 153 stored in the memory 152 that are run on the processor 151 when the system 100 is in use. The light head 140 houses a drive circuit 140 which receives control signals and in response to tire control signals causes the lights 130a-c of the traffic lights 102-108 to illuminate in a repeating sequence of cycles. Figure 3 shows an exemplary cycle which may be repeated endlessly in which each green light is illuminated a single time. Many other patterns may be used, perhaps with a green light illuminated more than once in a cycle. Associated with each traffic light 102-108 is a sensor apparatus shown in Figure 2. In this example the sensor apparatus comprises a radar sensor 160, four in total with one for each phase of the traffic light system. Note that only two a visible in Figure 2. The provision of radar sensors 160 are configured for detecting the presence of vehicles in a defined zone on a highway are well known and so will not be described in detail in this application. Tire radar sensors 160 may be attached to the pole next to the light head 120or in any other preferred position. The radar sensor should be oriented as to monitor the approach to the traffic light. Looking down onto the approach from a height allows both the first vehicle at or approaching a traffic light to be observed and also potentially a queue of vehicles behind that one. The zone of detection of the radar may extend in front of the signal for up to 60m. The radar sensors 160 may each output a sensor signal to the control unit which has a first value when it detects the presence of a vehicle at the traffic light and a second value when it does not detect the presence of a vehicle. The sensor signals from the four radar sensors are fed into the control unit 50. The control unit 150 processes the received sensor signals from each of the radar sensors 160 to determine the actual instantaneous level of traffic demand at each traffic light. Low instantaneous demand indicates that there are no vehicles wating to cross or verv few vehicles detected at the traffic light when a green light is illuminated by that traffic light indicating nobody was waiting to cross or that everyone waiting to cross has passed the traffic light before the end of the green light being illuminated. When there is high level of traffic demand the radar sensor may detect that there are vehicles waiting at the light for the whole green light illumination time. The control unit monitors the values of these sensor signals during the period that a specific phase is at green to determine whether this green duration needs to be increased or reduced in order to suit the underlying traffic conditions. . The control unit 150 combines the level of traffic demand information that is fed to it from the sensor apparatus or which the control unit calculates using data output from the sensor apparatus with information stored in the memory to determine an actual green light illumination time for each green light. The memory stores the following key pieces of information in the memory accessed by the control unit: - A Minimum Green Time - This is the absolute shortest period of time for which a green light is to be illuminated in each cycle. This may be unique for each traffic light. The actual green light time may not be set below this. -A Maximum Green Time which is the longest period of time for which a green light is to be illuminated. This may also be unique for each traffic light. Note that this varies every cycle and is not a user parameter at all. Ilie end of cycle calculation that looks at the quotas and requests for more or less green time on each phase, essentially just updates these maximums. -A green quota reserve value representing an amount of time that can be allocated across the set of traffic lights above their minimum green light time values; and-A mamixum green quota allocation time for each traffic light indicating a ring fenced amount of time that may always be allocated to a phase on a cycle. These parameters are set by an operative either locally at the traffic signals or via a remote connection. The parameters allow the relative priority of the phases to be expressed by the operative but then allow the system a lot of freedom to make best use of the available cycle time during operation. In use the control unit may set an actual illumination time for the green light of each traffic light equal to the minimum green time over a number of cycles, say 100 cycles of the traffic light. A cycle is a repeating pattern of illumination of the green light of the four traffic lights. During this time the sensor signals are monitored allowing the control unit to build up a estimate of the level of traffic deman for each traffic light indicative of how much traffic is trying to pass each traffic light over the monitoring period. This may be increased or decreased by the control unit on each cycle depending on whether the level of demand is seen to increase or decrease during the last cycle . In this example the control unit makes small adjustments to a demanded green time value at the end of each cycle, for example increment or decrement by 5s. The actual green light time is then set with a view to achieving these demanded times if at all possible using a quota system. Each adjustment is based on an evaluation made when the phase closes (transitions to red) of whether a phase received sufficient green time on the previous cycle or not. Over a period of time, the algorithm meanders it’s way towards the required green times. The control unit, based on the calculated demand values, extends the demanded green time for a traffic light whenever the demand indicates that a phase requires more green time. Once a demanded green time has been set for each light the control unit processes this value with the information stored in the memory to determine the actual green times that can be applied to each phase. The control unit accesses the memory and determines if the increased demanded green time is below tire value of thegreen quota allocation for a traffic light. If it is then the actual time is set to equal the demanded time. If the demanded duration is above the maximum alloated green quota time then time may be taken from the quota provided that there is quota time is remaining to be allocated. If it is not then the green light time is not extended and the actual time will be below the demanded time. The control unit determines the amount of quota time that each traffic light requires to take to meet the demanded time or to fall short of the demanded time for every light before then allocating any remaining quota time across the traffic lights according to the demanded green illumination times. Once time has been allocated for each traffic light, the actual green illumination time values are stored in the memory. The next time that the control unit determines a new value for the demanded green light times the actual green times may be recalculated. As noted earlier, in this example the control unit will perform this calculation once per cycle and set the allocations of time for the next cycle to use. This way, no phase gains an advantage over any other, apart from what is implicit in the values of the green quota allocation parameters that have been set. The value of the actual green illumination times is then encoded in the control signals sent by the control unit to each traffic light. The control unit may form part of a master unit and these control signals are used to generate instructions that are sent to every signal in real-time to tell the traffic light what it should be displaying (Red, Amber, Green). These may be sent wirelessly over radio. The signals themselves just do as they are told, second by second and do not need to receive and decode any control signals, just react to what they are told each second to display. At the end of each cycle, the control unit therefore sets the range of actual green times that will actually be used for the service of each phase . They vary constantly as the lights operate to dynamically control the flow of traffic. The diagram of Figure 4 shows the inputs and outputs of the calculation which results in the system determining the Min Green and Max Green parameters to be used on Phase A for tire next cycle. Once generated, these are used as described earlier whereby green is shown for the minimum period and then may be extended for any duration up to the maximum duration based on radar demand for a traffic light. In the following paragraphs the illumination of a green light on one traffic light is referred to as a phase. The lights are each illuminated at least one in a sequence to form a cycle, and the cycle then repeats over time. Setting a maximum time for the green light phase of each traffic light limits the maximum cycle time. The method steps involved in generating the value of Phase A Min Green and Phase A Max Green will now be described with reference to Figure 5 of the drawings. This shows the main steps performed to set tire actual green illumination time for one phase. These steps will be repeated for each traffic light in turn throughout the cycle and the repeated for each cycle indefinitely. Phase A Minimum Green time In a first step 200 this parameter is determined using a look-up table. The manner in which it is generated is not the subject of this patent application. This could vary for each phase. Minor roads may have a short min green, main roads could have a longer one. It would take a different value depending on what ‘priority’ the user selects for the phase. Phase A Actual Green illumination time One exemplary method used to calculate the actual Green illumination time for use by Phase A on the next cycle will now be described. As a starting point for the calculation, the system makes a determination 210 as to whether the actual Green time employed on the last cycle for Phase A was sufficient or not to meet the level of traffic demand. A decision 220 is made by the control unit to change the green time, either reduce the Phase A actual Green Time or to attempt to increase it or to keep it the same. In the event that the decision is to reduce the Actual Green illumination time, this is simply a case of removing 230 a fixed amount of time from Phase A actual Green time. Reductions in actual Green Time are always possible, subject to not falling below the Min Green illumination time value. Time that is removed is added back into the quota and the allocated quota time value is updated in the memory. Where the decision is made to attempt to increase the Actual Green time for phase A, the quota based scheme comes into play in order to determine which phases are given additional green time and how much they are permitted to use. The quota system allows maximum flexibility in the use of available green time whilst keeping to a firm limit on the overall cycle time of the signals. As explained below as well as allocating extra time to each traffic light from a pot called a quota the allocation may be done according to a set of rules where each traffic light has its own priority for allocation of time. Once the new actual green illumination demand time has been determined an appropriate control signal is generated by the control unit and fed to the receiver of each traffic light. Quota System operation In order to simply setup of a system of traffic lights using the quota system the user may enter a named priority level for each traffic light. This will enable the system to automatically populate the absolute minimum and absolute maximum time values and to set up quotas according to a set of predefined rules stored in the memory making set up very simple. Very high priority means the scheme designer requires that the signals are able to guarantee longer green times for this phase, typically a decision that is made because of the grade of road that this signal controls, or for other tactical or strategic reasons relating to keeping the local road network flowing and free of gridlock. The designer may apply a Very Low phase priority to a minor side road for which a decision is made that longer green times can only be provided if they are not required by the ‘main’ road. For this example, the end user may request the signals to apply the behaviour shown in Table 2 below: Phase Phase Priority A Very High B High C Low D Very Low In this example the control unit populates values for a max green quota allocation and an absolute maximum green time according to the rules shown in Table 3 below: Phase Max Green Quota allocation(secs) Absolute Maximum Green (secs) A 70 100 B 40 90 C 20 50 D 15 40 TOTAL 145 Table 3 5 What this data tells us is that Phase A is always entitled to set it's actual Green illumination time to 70s regardless of how much green time is being requested or used on the other phases (B, C, D). This quota amount of green time belongs fully to that phase. Additionally, Phase A is entitled to set it’s actual Max Green time value as high as 100s, never higher, as this set by the value of Absolute maximum green time in the memory. Use of green 10 time between 70s and 100s on Phase A is permitted only in the case where the sum of all the actual Green Times across all phases does not exceed the sum of all Max Green Quota values for all phases. In this example, this is 145s. Essentially a phase can exceed its ring fenced allocated quota by borrowing quota from other phases that are not fully using their allocation of quota (due to traffic levels not requiring the full 15 quota green time). Hie amount of unused quota across the system is referred to as the ’Green Quota Reserve’. It is shared fairly among phases that wish to make us of it. Examples The tables in this section provide example applications of the quota system. In these tables the following terms are used: Phase: This represents which phase of signals the timings relate to. A 2-way set of signals would have a phase A and B. The examples are for a 4-way system of the kind shown in Figure 1 hence we have phases A, B, C, D. Fixed Parameters: These are parameters entered by the operative (by means of the human readable phase priority select e.g High Priority). They vaiy only when changed by the user or according to a user-defined schedule. Demanded Maximum Green time: This is the ideal amount of green time that the signals would apply on the next cycle due to the level of traffic demand if there were not restriction on them due to the quota system Actual green illumination time- the actual amount of time that a green light is illuminated during a cycle which may be less than the demanded max green time due the restricts improsed by the quota system. Taken from Quota: This has a value up to the amount of time that the phase can be granted in full due to it being part of the allocated quota for that phase. Outstanding Requested: The difference between the Requested Max Green and Taken from Quota’. Negative values are set to zero. Quota Reserve: The amount of quota (across all phases) that is unused once the control unit has assigned the ring fenced quota time needed by each phase. Quota Reserve Allocation: Shows how the unused quota is allocated equally to phases that request green time in excess of their quota. The following case shows an example where some phases request access to green time in excess of their quota and where such requests are all granted in full because the total quota value is not exceeded as shown in Table 4. Fixed Parameters Demanded Green Time Actual Green time Calculation Values Calculated actual Green time Phase Max Green Quota Absolute Max Green Taken from Quota Outstanding Requested Quota Reserve Allocation A 70 100 90 70 20 20 70+20 = 90 B 40 90 20 20 0 0 20+0 = 20 C 20 50 15 15 0 0 15+0 = 15 D 15 40 15 15 0 0 15+0 = 15 TOTAL 145 120 140 Green Quota Reserve 25 Table 4 Fixed Parameters Demanded Green time Actual Green Time Calculation Values Calculated actual Green time Phase Max Green Quota Absolute Max Green Taken from Quota Outstanding Requested Quota Reserve Allocation A 70 100 100 70 30 70+0 = 70 B 40 90 80 40 40 V 40+0 = 40 C 20 50 45 20 25 ; J 20+0 = 20 D 15 40 35 15 20 15+0 = 15 TOTAL 145 145 145 Green Quota Reserve 0 In contrast, table 5 shows an example where some phases request access to green time in excess of their quota and where such requests cannot be granted as the total quota value would otherwise be exceeded. 5 The following case shows an example where some phases request access to green time in excess of their quota and some below their quota Unused quota is redistributed equally among the other phases that require use of it. See table 6 below: Fixed Parameters Demanded Green time Actual Green Time Calculation Values Calculated Actual Green time Phase Max Green Quota Absolute Max Green Taken from Quota Outstanding Requested Quota Reserve Allocation A 70 100 40 40 0 0 40+0 = 40 B 40 90 20 20 0 0 20+0 = 20 C 20 50 55 20 35 50 / 2 = 25 20+25 = 45 D 15 40 45 15 30 50 / 2 = 25 15+25 = 40 TOTAL 145 95 145 Green Quota Reserve 50 10 Table 6.

Claims

1 .A traffic light system comprising at least two traffic lights which are operated in a repeated sequence of cycles, each traffic light comprising a green light which indicates that vehicles may pass the traffic light when the green light is illuminated,at least one control unit that is arranged to generate a respective control signal for each traffic light that determines a requested actual amount time for which the green light is illuminated within the cycle, each traffic light being responsive to the respective control signal to illuminate the green light of that traffic light for the requested amount of time within a cycle,and a memory which stores data for use by the control unit to set the duration of the requested illumination of the green lights, the stored data comprising :a minimum green time value indicative of the minimum duration for illumination of a green light of each traffic light during a cycle; anda green quota reserve value indicative of an overall amount of additional time for illumination of the green lights over the minimum green time value during a cycle that can be allocated across the traffic lights,and in which the control unit is arranged to set the actual demanded green light time by allocating time from green quota reserve across the traffic lights as a function of a level of traffic demand at each traffic light such that the actual minimum time value for each light is met or exceeded and that the total additional time allocated across all of the green lights of the system does not exceed the value of the green quota reserve.

2. A traffic light system according to claim 1 in which the memory stores a value representing the green quota reserve that is fixed by an operator during installation or commissioning of the system.

3. A traffic light system according to claim 1 or claim 2 in which the memory stores a maximum green value for the green light of each of the traffic lights which represents the maximum duration of illumination that the control unit must not exceed when allocating time from the green quota reserve.

4. A traffic light system according to any preceding claim in which the memory stores a unique minimum green time duration for each traffic light.

5. A traffic light system according to any preceding claim configured to assign a priority to each of the traffic lights such that the assignment of time from the green quota reserve to those with a higher priority differs from those with a lower priority'.

6. A traffic light system according to claim 5 in which the control unit assigns a green quota value to each traffic light representing the maximum amount of time that is guaranteed to be available to be assigned to that traffic light from the green quota reserve.

7. A traffic light system according to claim 6 in which a higher priority' traffic light is assigned a higher green quota time than a lower priority traffic light.

8. A traffic light system according to any one of claim 6 or 7 in which the control unit is arranged to only assign additional time to a green light time of a traffic light above the green quota if there is unused quota time that has not been assigned to the other traffic lights.

9. A traffic light system according to any preceding claim in which the control unit sets the actual green illumination time for each traffic light at the end of each complete cycle.

10. A traffic light system according to claim 9 in which the control unit reduces the actual maximum green time for any phase which was not experiencing a traffic demand up to the point of it’s set green time value during the immediately preceding cycle.

11. A traffic light system according to any preceding claim in which the control unit processes during a cycle of the traffic lights one or more of the following:-the traffic level of demand signal associated with that traffic light;-a minimum green light illumination time assigned to the traffic light;-a maximum green light illumination time assigned to the traffic light;- the amount of quota time assigned to each of the other lights at the time the calculation is being made;-the allocated minimum amount of quota time that each of the other traffic light is entitled to take; and- the actual green light illumination time allocated to the traffic light on the last cycle.

12. A traffic light system according to any preceding claim that further comprises a sensor apparatus associated with each traffic light that detects the presence of vehicles at the approach to the associated traffic light and outputs a traffic level of demand signal to the control unit that is used when allocating quota time.

13. A method of controlling at least two traffic lights each traffic light comprising a green light, the method comprising operating each of the lights in sequence whereby the duration of each green light being illuminated within a cycle of a sequence is set by the following steps:(a) determining a demanded illumination duration for a green light of each of the traffic lights as a function of a level of traffic demand for each traffic light received from a sensor apparatus:(b) assigning a minimum green time value to each traffic light indicative of the minimum duration for illumination of a green light of a traffic light during a cycle;(c) setting a green quota reserve value indicative of an overall amount of additional time for illumination of the green lights over the minimum green time value during a cycle that can be allocated across tire traffic lights, and(d) allocating time from the green quota reserve across the traffic lights as a function of the level of traffic demand for each traffic light to set an actual maximum green illumination time for each green light such that the minimum illumination time value for each light is met or exceeded and that the total additional time allocated to the traffic lights from the green quota reserve does not exceed the value of the green quota reserve.

14. A method according to claim 13 in which the steps (b) and (c) are performed once during installation of the traffic lights or periodically set or adjusted by an operator.

15. A method according to claim 13 or claim 14 in which die steps (a) and (d) are performed during operation of the traffic lights, for example once each cycle at the end of a cycle or start of a cycle.

16. A method according to any one of claims 13 to 15 comprising stepping each value of actual green time up by a preset amount of time when the level of traffic demand is increased relative to that used when the actual green time was last set.

17. A method according to any one of claims 13 to 16 comprising stepping each value of actual green time down by a preset amount of time when the level of traffic demand is decreased relative to that used when the actual green time was last set. This preset amount may be a value between 1 second and 10 seconds, preferably 5 seconds. Hie timing of the lights will therefore only vary slowly regardless of how rapidly the demand changes.

18. A method according to any one of claims 13 to 17 which further comprises setting a priority for each traffic light and step (d) comprises allocating time from the green time reserve quota as a function of the assigned priorities.

19. A method according to claim 18 comprising setting a green quota time value for each traffic light representing an amount of time that light may always be able to take from the quota when determining the actual green light illumination time.27

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