Patent Description:
Event-responsive traffic lights enable a flexible traffic light control adapted to a detected lane-related event at an intersection of lanes in a transportation network. To this end, a controller which switches the traffic lights between different switching states (also called "phases") according to a specific phase sequence in cycles may detect one or more lane-related events such as the presence or arrival of pedestrians, trams, buses, emergency vehicles, etc., and change to switching the traffic lights according to a different phase sequence to take into account the traffic demand related to the event.

For instance, at an intersection of a car lane and a pedestrian crosswalk with a push-button for pedestrian detection, the controller may change - upon button pushing - from a first phase sequence comprising a single first switching state with a green traffic light for the car lane and a red traffic light for the pedestrian crosswalk to a second phase sequence comprising the first switching state and a second switching state of reversed traffic light colours to allow for pedestrian crossing in the current cycle. When the pedestrian/s have crossed the lane, the controller may change back to the first phase sequence to allow for undisturbed car traffic in a subsequent cycle and wait for a further button pushing to induce a further phase sequence change.

However, most often the phase sequences that are statically stored in and available to the controller do not satisfy the current or near-future traffic situation in the transportation network. For instance, in the above exemplary intersection of a pedestrian crosswalk crossing a car lane, the duration and the number of occurrences of the pedestrian traffic-allowing second switching state can affect the overall traffic situation in the transportation network. On the one hand, if this duration is too long or if there are too many occurrences, cars at the intersection wait unnecessarily, long car queues gradually form, slowing down traffic at neighbouring intersections, and ultimately traffic jams arise. On the other hand, if this duration is too short or if there are too few occurrences, pedestrian queues form and pedestrians tend to dangerously cross the crosswalk in a non-allowing traffic light switching state. Empirical as well as theoretical optimisations of the phase sequences that are statically stored may be performed, e.g., for average traffic flows on the lanes forming the intersection, which, however, still cannot cope with the traffic fluctuations arising over time.

Consequently, traffic lights are still controlled according to unsuitable phase sequences resulting in retarded traffic flows, long queue lengths, traffic jams and dangerous unallowed lane crossings.

<CIT> discloses a system and a method for adaptive and/or autonomous Traffic Control. In particular, the method includes receiving data regarding travel of vehicles associated with an intersection, using neural network technology to recognize types and/or states of traffic, and using the neural network technology to process/determine/memorize optimal traffic flow decisions as a function of experience information.

It is an object of the present invention to provide a method and a server for controlling traffic lights which allow for an improved event-responsive traffic light control.

To this end, in a first aspect the invention provides for a method of controlling event-responsive traffic lights at an intersection of lanes in a transportation network by means of.

The method of the invention is based on a repeated update of the set of phase sequences used by the controller for traffic light switching. The server repeatedly determines the current or near-future traffic flow on the lanes of the intersection from the traffic flows measured by the sensor/s, identifies that candidate set of phase sequences which fits the determined traffic flow and sends that candidate set to the controller which, in turn, receives, stores and uses the same until the next update. As a result, the controller always uses a set of phase sequences which currently minimises the traffic flow dependent cost measure. The set of phase sequences is thus always adapted to the current or near-future traffic flow in the transportation network, in particular in the vicinity of the intersection.

The candidate sets, from which the set of phase sequences used by the controller is repeatedly identified, are generated with a specific time-slotted structure: Each candidate set has its own grid of ordered time slots common to all phase sequences contained therein. Therefore, a change from one phase sequence to another phase sequence within a specific candidate set can be easily carried out at candidate set specific phase transition times ("anchor points") between each two time slots and, hence, all phase sequences in a candidate set are per design compatible with one another with respect to timing. This allows the controller to detect lane-related events occurring in a time slot and to change at the transition time (anchor point) to the next time slot to that phase sequence of the (currently stored) optimal set which fits to the detected events.

Summing up, the inventive methods allows for a traffic light control according to a set of phase sequences which is adapted to the current or near-future traffic flows in the transportation network such that traffic flows are accelerated, queue lengths reduced, traffic jams minimized and unallowed pedestrian lane crossings prevented.

In a favourable embodiment each of said intervals lasts at least two cycles. Thereby, the frequency of traffic flow determination will be decoupled from the controller cycles and can, e.g., be adapted to traffic flow measurement constraints. Furthermore, optimal candidate set identification and sending and, hence, communication bandwidth needs therefor will be reduced.

In a preferred embodiment the candidate sets are generated such that, in all candidate sets, a specific phase occupies only time slots which each have a minimum duration. By selecting only time slots of at least a minimum duration for a specific phase, constraints such as the minimum traversing time for pedestrians, bikers, cars, etc. required for that phase can be implemented in a simple manner and a generation of unsuitable candidate sets can be avoided. Or, seen from another perspective, the solution space of available candidate sets to be considered by the server is reduced and the optimal candidate set can be identified faster and at a lower computational cost.

The cost measure which is employed in identifying the optimal set of phase sequences can be any suitable traffic measure considering, e.g., locations, counts, speeds, accelerations etc. of traffic participants. Favourably, the cost measure is an average waiting time or an average queue length. These quantities are particularly impactful and, thus, suited to accelerate traffic flows, reduce queue lengths, and avoid traffic jams as best as possible. When the cost measure is an average waiting time, it is preferably calculated as <MAT> with.

Such a cost measure calculation is particularly easy to implement and fast in execution such that the candidate set which minimises the cost measure can be quickly identified.

The traffic flows on the lanes may in principle be determined only from the traffic flows measured on that lanes which form the intersection of interest. Alternatively, the traffic flows on these lanes may be determined by utilising further information on the transportation network. To this end, it is beneficial when the server is connected to a further sensor for measuring a traffic flow on a further lane in the transportation network, and when, in said step b) of determining the traffic flows, the traffic flow on the further lane is as well taken into account. Thereby, the near-future traffic flow on the lanes that form the intersection can be more accurately determined and considered to identify and send the most suitable candidate set to the controller. Moreover, when the inventive method is carried out for all intersections in the transportation network, the traffic light switching in the whole transportation network may be optimised and traffic interrelations at different intersections may be taken into account more accurately the more traffic flows are measured throughout the entire transportation network.

In a second aspect the invention provides for a central server for controlling event-responsive traffic lights at an intersection of lanes in a transportation network, the server being connectable to a controller that switches the traffic lights in cycles and, in each cycle, in a sequence of phases, each phase corresponding to a different switching state of the traffic lights and allowing traffic flow on none, one or more of the lanes, and being connectable to one or more sensors that measure traffic flows on the lanes, wherein the server is configured to:.

to repeat the steps b) - d) in successive intervals, each interval lasting at least two cycles.

As to the inventive server, the same benefits, advantages and preferred features apply as were discussed for the method of the invention.

The invention shall now be described in more detail by means of exemplary embodiments thereof under reference to the enclosed drawings, in which show:.

<FIG> shows a transportation network <NUM> and an intersection <NUM> of lanes <NUM> - <NUM> in the transportation network <NUM>. The transportation network <NUM> can comprise any type of lanes such as car lanes (lanes <NUM> and <NUM>), bus lanes (lane <NUM>), tramways (lane <NUM>), pedestrian crossings (lanes <NUM> and <NUM>), bike lanes (not shown), railways (not shown), mixed lanes (not shown), etc. The intersection <NUM> may be formed by any crossing, removal (e.g. road narrowing) or opening (e.g. road expansion) of lanes.

The traffic at the intersection <NUM> is regulated via event-responsive traffic lights <NUM> - <NUM> which are controlled by a local controller <NUM> via a wired or wireless control path A. The controller <NUM> switches the traffic lights <NUM> - <NUM> in cycles of a cycle duration Tc and, in each cycle, according to a sequence <NUM>j of phases Pn (<FIG>). Each phase P<NUM>, P<NUM>,. , generally Pn, corresponds to a specific switching state of the traffic lights <NUM> - <NUM> and, thus, allows traffic flow on none, one or more of the lanes <NUM> - <NUM>, e.g., a first phase P<NUM> corresponds to a switching state of the traffic lights <NUM> - <NUM> being green and the remaining traffic lights <NUM> - <NUM> being red, which allows for traffic flow on the lanes <NUM> - <NUM> and <NUM> as indicated by the solid arrows in <FIG>.

For event-responsive control of the traffic lights <NUM> - <NUM> according to a traffic demand at the intersection <NUM>, the controller <NUM> detects lane-related events indicating a traffic demand and controls the traffic lights <NUM> - <NUM> in dependence thereon. To detect lane-related events such as the presence or arrival of pedestrians <NUM>, a tramway, a bus, an emergency vehicle etc., the controller <NUM> is connected or connectable to event detectors like pedestrian push-buttons <NUM>, a tramway detector <NUM>, a bus detector <NUM>, a wireless communication device carried by an emergency vehicle (not shown) etc., via a wired or wireless detection path B.

To control the traffic lights <NUM> - <NUM> event-responsively, the controller <NUM> stores, in a memory <NUM>, a set <NUM> of phase sequences <NUM><NUM>, <NUM><NUM>,. , <NUM>J, generally <NUM>j (j = <NUM>. J), (<FIG>) and changes between the phase sequences <NUM>j upon detection of a lane-related event.

As can be seen in <FIG>, each phase sequence <NUM>j of the set <NUM> has ordered time slots S<NUM>,j, S<NUM>,j,. , SI,j, generally Si,j (i = <NUM>. Each time slot Si,j is occupied by a respective one of the phases Pn. Time slots Si,j of the same order i have the same duration Ti. Hence, all first time slots S<NUM>,<NUM>, S<NUM>,<NUM>,. , S<NUM>,J have the same duration T<NUM>, all second time slots S<NUM>,<NUM>, S<NUM>,<NUM>,. , S<NUM>,J have the same duration T<NUM>, and so on, to facilitate the change between phase sequences <NUM>j at the transitions ("anchor points") between one time slot Si,j and the next time slot Si+<NUM>,j.

For instance, without any detection of a lane-related event, the controller <NUM> may switch the traffic lights <NUM> - <NUM> according to the first phase sequence <NUM><NUM> of <FIG> only between two phases P<NUM> and P<NUM> and thereby first allow traffic on the lanes <NUM>, <NUM> and <NUM> in the first phase P<NUM> (solid arrows in <FIG>) for two time slots S<NUM>,<NUM> and S<NUM>,<NUM> of durations T<NUM> and T<NUM>, and then allow car traffic on the lanes <NUM>, <NUM> and <NUM> in the second phase P<NUM> (dotted arrows in <FIG>) for three time slots S<NUM>,<NUM>, S<NUM>,<NUM> and S<NUM>,<NUM> of durations T<NUM>, T<NUM> and T<NUM>.

However, upon detection of a lane-related event, e.g. a detection that pedestrians <NUM> have activated the push-button <NUM> within the first time slot S<NUM>,<NUM>, the controller <NUM> changes from the first phase sequence <NUM><NUM> to another phase sequence <NUM>j (see block arrow <NUM> in <FIG>) that comprises a third phase P<NUM> in which the traffic lights <NUM>, <NUM> are green, to allow for a pedestrian crossing (dashed arrow in <FIG>) within the cycle duration Tc. Depending on activations of further detectors <NUM>, <NUM> the controller <NUM> may then, e.g., change again from the j-th sequence <NUM>j to another sequence <NUM>J (see block arrow <NUM> in <FIG>) which, e.g., comprises a fourth phase P<NUM> allowing for tramway traffic and a fifth phase P<NUM> allowing for bus traffic, etc..

Thus, in general, the controller <NUM> monitors the detected lane-related events within each time slot duration Ti or, e.g., in a "look back" time window W preceding the end of the time slot duration Ti, determines that phase sequence <NUM>j from the set <NUM> which fits the detected lane-related events, and changes thereto at the end of the time slot duration Ti, i.e. at a phase transition time ("anchor point") ti,i+<NUM> (in <FIG>: t<NUM>,<NUM>, t<NUM>,<NUM>, t<NUM>,<NUM>, t<NUM>,<NUM>).

However, the set <NUM> of phase sequences <NUM>j stored in the controller <NUM> might not suit well the current traffic situation in the transportation network <NUM>, e.g. green phases for passengers and/or cars might be too long or too short, too rare or too frequent within each cycle. To overcome this problem, a central server <NUM> is connected to the controller <NUM> and configured to update the set <NUM> used by controller <NUM> on the basis of the current or near-future traffic flow on the lanes <NUM> - <NUM>. A method <NUM> for updating the controller <NUM>, which is carried out by a system <NUM> formed by the controller <NUM> and the central server <NUM>, shall now be described with reference to <FIG> and <FIG>.

The central server <NUM>, carries out a first part <NUM>s of the method <NUM> comprising steps a) - d) and the controller <NUM> carries out a second part <NUM>c of the method <NUM>.

In the first step a) of the method <NUM> the server <NUM> generates candidate sets <NUM><NUM>, <NUM><NUM>,. <NUM>K, generally <NUM>k, of phase sequences <NUM>j,k (the additional index k denoting the candidate set dependency). One of candidate sets <NUM>k will later be used as the set <NUM> by the controller <NUM>.

As illustrated in <FIG>, each candidate set <NUM>k is generated with its own time slot durations Ti and phase transition times ti,i+<NUM> within the cycle duration Tc: In each candidate set <NUM>k all phase sequences <NUM>k,j have a same number (here: five; alternatively more or less) of ordered time slots Si,j,k (the additional index k denoting the candidate set dependency), and time slots Si,j,k of the same order, i.e. with the same index i, have the same duration Ti as described above. Different candidate sets <NUM>k, however, differ at least in the duration Ti of a time slot Si,j,k of a specific order and, hence, in at least one phase transition time ti,i+<NUM> within the cycle duration Tc. Optionally, different candidate sets <NUM>k may further differ, e.g. in the number of time slots Si,j,k or in the phases Pn occupying the time slots Si,j,k.

Each time slot Si,j,k is occupied by a respective phase Pn, and the selection of phases Pn occupying all time slots of a phase sequence <NUM>j,k can either be carried out by combinatorics, e.g., by choosing all possible combinations of phases Pn for the time slots Si,j,k, or empirically by selecting only those sequences of phases <NUM>j,k that allow for a smooth traffic without a blockage of the intersection <NUM>. To reduce the number of candidate sets <NUM>k and to employ only suitable phase sequences <NUM>j,k the candidate sets <NUM>k may optionally be generated such that a specific phase Pn may only occupy a time slot Si,j,k of a minimum duration Ti. For instance, the phase P<NUM> allowing for pedestrian crossing may require a minimal allowing ("green") time for an actual pedestrian crossing and, thus, only occupy time slots Si,j,k whose duration Ti is larger than that minimal green time.

In the second step b) of the method <NUM> the server <NUM> determines the traffic flows on the lanes <NUM> - <NUM>. To this end, the server <NUM> is connected to one or more sensors <NUM> - <NUM> for measuring the traffic flows on the lanes <NUM> - <NUM>, receives the measured traffic flows therefrom via wired or wireless paths C, and processes the measured traffic flows. Each of the sensors <NUM> - <NUM> may be any traffic flow sensor, such as an inductive loop, a radar, an active or passive infrared sensor, a video sensor, a sensor communicating with mobile phones or vehicle carried devices indicating their locations, etc..

In a first variant the server <NUM> may simply take the measured flows as determined flows to determine the current traffic flows on the lanes <NUM> - <NUM>.

In a second variant the server <NUM> may predict the traffic flows on the lanes <NUM> - <NUM>. In this case, the server <NUM> may optionally be further connected to one or more further sensors <NUM> for measuring the traffic flows on further lanes <NUM> which do not form the intersection <NUM>. These additional traffic flows may then be taken into account to predict the near-future traffic flow on the lanes <NUM> - <NUM> forming the intersection <NUM>, e.g. utilising a traffic flow model.

In the third step c) of the method <NUM> the server <NUM> calculates a cost measure Dk for each candidate set <NUM>k in dependence on the traffic flows determined in step b) and in dependence on the durations Ti of the time slots Si,j,k in the candidate sets <NUM>k generated in step a). The cost measure Dk may be any measure quantifying traffic flow such as an average queue length or an average waiting time at the intersection, etc. In an exemplary embodiment the cost measure Dk is an average waiting time and calculated according to <MAT> with.

The cumulative determined traffic flow qi,j,k is the sum of all traffic flows determined for those lanes that have a respective "traffic-allowing" ("green") traffic light <NUM> - <NUM> in the phase Pn occupying the respective time slot Si,j,k. For example, in the phase P<NUM> of <FIG> these are the lanes <NUM> - <NUM>. The cumulative given saturation flow Si,j,k is a given design parameter of the transportation network <NUM> which indicates the sum of the maximally possible ("saturation") traffic flows for said lanes that have a respective traffic-allowing (green) traffic light <NUM> - <NUM> in said phase Pn occupying said respective time slot Si,j,k (in the phase P<NUM> of <FIG>: the lanes <NUM> - <NUM>).

In case the candidate sets <NUM>k do not differ in the phases Pn occupying the time slots Si,j,k, equation (<NUM>) may be simplified to read <MAT> with.

In a subsequent step d) of the method <NUM>, the server <NUM> identifies that candidate set <NUM>k,opt for which the smallest cost measure Dk has been calculated in step c) and sends that candidate set <NUM>k,opt to the controller <NUM>.

Then, in the second part <NUM>c of the method <NUM>, the controller <NUM> receives the candidate set <NUM>k,opt sent in step d), stores that candidate set <NUM>k,opt as the set <NUM> of phase sequences <NUM>j, and uses the same as mentioned above with reference to <FIG>. The set <NUM> has thus been updated by the candidate set <NUM>k and thereby been adapted to the current or near-future traffic situation at the intersection <NUM>.

For a repeated update of the controller <NUM>, the server <NUM> carries out steps b) to d) repeatedly in successive intervals as indicated by the loop <NUM> in <FIG>. Each of the successive intervals lasts, e.g., at least two cycle durations Tc for a regular update of the set <NUM> with a low required bandwidth for communications between the server <NUM> and the sensors <NUM> - <NUM>, on the one hand, and the server <NUM> and the controller <NUM>, on the other hand.

While the method <NUM> has been described exemplarily for a single intersection <NUM>, it shall be noted that the server <NUM> can (and typically will) carry out the steps a) to d) for more intersections of the transportation network <NUM>, to generate candidate sets for further intersections, determine traffic flows on further lanes, and update the controller/s which switch the traffic lights at further intersections according to the current or near-future traffic situation in the whole transportation network <NUM>.

Claim 1:
A method of controlling event-responsive traffic lights (<NUM> - <NUM>) at an intersection (<NUM>) of lanes (<NUM> - <NUM>) in a transportation network (<NUM>) by means of
a controller (<NUM>) which switches the traffic lights (<NUM> - <NUM>) in cycles and, in each cycle, in a sequence (<NUM>j) of phases (Pn), each phase (Pn) corresponding to a different switching state of the traffic lights (<NUM> - <NUM>) and allowing traffic flow on none, one or more of the lanes (<NUM> - <NUM>), wherein the controller (<NUM>) has a memory (<NUM>) storing a set (<NUM>) of phase sequences (<NUM>j) and is configured to change from one phase sequence (<NUM>j) of the set (<NUM>) to another phase sequence (<NUM>j) of the set (<NUM>) upon detection of a lane-related event, and by means of
a central server (<NUM>) connected to the controller (<NUM>) and to one or more sensors (<NUM> - <NUM>) that measure traffic flows on the lanes (<NUM> - <NUM>),
the method (<NUM>) comprising, in the server (<NUM>):
a) generating candidate sets (<NUM>k) of phase sequences (<NUM>j,k), wherein in each candidate set (<NUM>k) all phase sequences (<NUM>j,k) have a same number of time slots (Si,j,k) in the same order, each time slot (Si,j,k) is occupied by a respective phase (Pn) and time slots (Si,j,k) of the same order have the same duration (Ti), and wherein each candidate set (<NUM>k) differs in at least one duration (Ti) of a time slot (Si,j,k) of a specific order;
b) determining the traffic flows on the lanes (<NUM> - <NUM>) by means of the one or more sensors (<NUM> - <NUM>);
c) calculating a cost measure for each candidate set (<NUM>k) on the basis of the determined traffic flows on the one hand and the durations (Ti) on the other hand; and
d) sending the candidate set (<NUM>k,opt) with the lowest cost measure to the controller (<NUM>);
in the controller (<NUM>), receiving and storing (<NUM>c) the sent set (<NUM>k,opt) in the memory (<NUM>) as the set (<NUM>) of phase sequences (<NUM>j); and
repeating steps b) - d) in successive intervals.