Vehicle to traffic signal or AWTCS
The purpose of communications interactions between vehicles and traffic signals is to provide signal priority for public transport services. These systems allow vehicles to operate along routes with minimal stoppages and are particularly useful in city centres where congestion is often a problem. A variety of systems are available to achieve signal priority, with the most common being roadside detectors and sub-surface detectors. Typically subsurface detectors, usually inductive-loop detectors, are used in situations where traffic lanes are reserved for public transport vehicles. In these systems the vehicle does not communicate with the traffic signal control directly, but is sensed by the detector which then interacts with the signal control unit to request priority.
In a typical roadside detector unit system there is direct communication between the roadside detector/traffic-signal control unit and the vehicle. In this system an in-vehicle transmitter broadcasts a radio signal to identify itself. This signal is picked up by a receiver in the roadside unit when the vehicle comes into range, which then knows that the vehicle is approaching and requesting priority. When the vehicle arrives at the junction it will give a further message that it has arrived and is requesting priority. Priority is usually provided by extending a green light or shortening a red light for the on-coming vehicle. Once the vehicle has passed through the junction it will transmit a further message to indicate that it is clear. Other technologies that can be used for traffic signal priority include infrared systems and GPS/AVL systems. In infrared systems signal priority is also provided by extending or shortening signal phases. In this type of system an in-vehicle unit emits an infrared signal which is received by a detector mounted on the traffic light. This signal is converted into an electrical impulse and is sent to the traffic light control unit which then alters the signal timing to award priority. GPS/AVL systems award priority in the same way as the systems previously discussed but request it based on vehicle location information. An in-vehicle radio transmitter transmits speed, location and heading information to a receiver mounted on the traffic light updating the information approximately every second. This radio transmitter is connected to the vehicles GPS system which provides the real-time location data.
Priority can be provided on a number of levels regardless of the system implemented to achieve it. In some cases priority is only given when vehicle are running behind schedule and in others priority is always given. In addition to this, priority can be given in a hierarchical structure, for example trams can be awarded priority over busses and busses awarded priority over general traffic.
Communications systems integrating vehicles and traffic signals are predominantly used as operations management tools and to aid drivers for schedule adherence support. They also function as traffic management tools to minimise the impacts of congestion and to reduce unnecessary stopping time along routes.
Benefits and cautions
All traffic signal priority technologies minimise the impacts of congestion on public transport services and aim to award priority in a manner that also causes minimal disruption and frustration for other road users. The effectiveness of each type of system in achieving this depends on the speed of information transfer, between the vehicle and traffic control unit, to warn of approach. Effectiveness also depends on the avoidance of obstacles which could prevent the vehicle from reaching the junction in reasonable time. For this second reason signal priority schemes are most suited to dedicated lanes for public transport services, although this is not a requirement for an effective system. AVL/GPS systems are advantageous in that they can effectively pre-warn signal control units of estimated arrival time and this allows for smoother integration of the priority signal into the standard signal phase. However this type of systems needs to be able to continuously and reliably update the signal control unit very regularly without GPS service disruption. This could be problematic in dense urban areas with many large structures that could disrupt GPS signal.
Relevant case studies