Supporting Technologies for Signal Priority

Detection Systems

According to BMVBW & VDV (2001), detection systems for signal priority must fulfil the following requirements:

• Possible to transmit requests over long distances (up to 500 m), • High reliability of system operation,

• Ability to perform rapid control,

• Appropriate for checking buses at any arrival speeds, • Short time frame for registering (not exceed one second), • No requirements of drivers’ intervention, and

In Germany, the most common detection systems used for signal priority consist of inductive systems, beacon systems, and satellite-based detection systems. Based on BMVBW & VDV (2001), the summary of these systems is given in the following parts.

Inductive Systems

An inductive loop detector consists of a wire loop installed under road pavements, power supplies, and evaluation circuits. By measuring and evaluating the change in the magnetic alternating field, inductive systems can be used as detection systems for signal priority.

In practice, depending on specific situation, the requirements of inductive systems can be different as follows:

• If buses operate on their separate lanes, the presence and location of requesting buses can be determined by normal inductive systems.

• If buses operate on mixed traffic lanes, inductive loop detectors with the special evaluation must be used to distinguish different types of vehicles in mixed traffic flow. • If data transmission from buses to controllers is required, additional on-board transducers

must be needed. The integrated on-board information system (IBIS) can be used to provide necessary information.

However, inductive loops are generally not adequate when buses operate in mixed traffic conditions. The main reason is that they possibly detect the incorrect requests disturbed by private traffic and they can only provide minor information content for some comprehensive measures of signal priority. [RiLSA—FGSV (1992)]

Beacon Systems

Beacon systems are being commonly used for detection systems of signal priority in Germany. The operation of these systems is based on transceiver principles, using infrared radiation (IR) as the transmission medium (see Fig. 10).

Figure 10: Beacon systems for signal priority [Source: BMVBW & VDV (2001)]

Infrared beacon systems consist of receivers and transmitters equipped both on vehicles (on- board devices) and on roads (route devices or local beacons). Basically, a direct light-of-sight connection between on-board devices and local beacons must be required. On-board devices transmit their signals continuously, while local beacons are activated only by passing buses. When a bus enters the detection areas of a local beacon, signals from on-board device are sent to the local beacon. Depending on the type of beacon systems, bus requests can be sent directly from the bus to the signal controller via radio, or indirectly through the local beacon linked to the controller by cable.

Compared to inductive systems, beacon systems enable to exchange more types and volumes of information (e.g. vehicle presence, direction, line recognition, degree of occupancy, etc.) between vehicles and roadside installation. [RiLSA—FGSV (1992)]

Satellite-Based Detection Systems

Satellite-based detection systems consist of the on-board unit (the GPS receiver in conjunction with the on-board computer) and the receiver at the signal controller. These systems can be integrated into Intermodal Transport Control System (ITCS). Based on Global Positioning System (GPS) signals, the real-time location of buses is detected for the use of signal priority (see Fig. 11).

Figure 11: Example of satellite-based detection systems for signal priority [Source: Freie und Hansestadt Hamburg Baubehörde (2000)]

When a bus approaches a predefined detection zone (or virtual detection zone) before traffic signals, the on-board unit will generate bus requests automatically. Then the requests are sent from the bus to the signal controller via radio transmission. In order to implement signal priority efficiently, the location of the bus must be determined as precise as possible. However, the error of GPS location detection normally ranges between 50 and 60 m, thereby this detection is not sufficient in accuracy for registering signal priority. To deal with this problem, Differential

Global Positioning System (DGPS) with the error ranging between 5 and 10 m is currently used as satellite-based detection systems for signal priority.

In the United States, according to Intelligent Transportation Society of America (2005), the most common detection technologies used for signal priority consist of (1) hard-wired loop detection, (2) light-based detection, (3) sound-based detection, (4) radio-based detection, and (5) satellite- based detection. The major attributes of these technologies are summarised as follows:

• Hard-wired loop detection consists of a transponder attached to underside of a bus, and a detection receiver that integrates with the signal controller. The advantage of loop detection systems includes the utilisation of existing loop detectors, reliable detection, stability with weather conditions, and non-requirements of line-of-sight or visibility. Moreover, the priority cancellation is easily implemented by inserting another loop after the stop line. However, the limitation of these systems is that they need to be placed and maintained appropriately.

• Light-based detection is one of the most widely used detection systems for signal priority, and it has been well tested for many years. This system contains an infrared strobe emitter located on a bus, and an infrared detector placed at the local intersection. However, this detection type requires line-of-sight between emitters and detectors. Thereby, its main disadvantage involves the obstruction (e.g. obstructions caused by trees, traffic signs), weather conditions, and potential false detection with nearby intersections.

• Sound-based detection includes audible and non-audible detection. Audible detection is mainly used for emergency vehicles, and it is not practical for transit signal priority. Otherwise, non-audible detection (digital sound wave recognition system) can be used for this purpose. The advantage of this detection system is that it does not depend on light-of-sight and visibility issues between the emitter and the detector. However, this system has some problems with the possibility of false detection, lack of vehicle identification, and logging capabilities.

• Radio-based detection system includes two types. The first type consists of radio frequency (RF) transponders mounted on buses and RF tag readers installed at upstream of signalised intersections. The second type embodies antennas and receivers mounted at intersections to receive radio signals from radio transmitters located on buses. The data (incl. identification number, travel direction, time, date, and duration) can be transmitted by radio signals to receivers. The advantage of this detection system is that it does not depend on light-of-sight and visibility issues. However, the costs for equipment are relatively high, and this system often requires suitable curb-side locations for mounting necessary equipment.

• Satellite (GPS)-based detection basically has two major types. The first type of GPS- based detection consists of the in-vehicle unit and the field unit located at the signal controller. The real-time location of buses is updated constantly by using a GPS-based AVL (automatic vehicle location) system. Based on vehicle schedule deviation,

passenger load, and other factors, data information is sent from the in-vehicle unit to the field unit. Then the priority message with regard to vehicle or route number, vehicle approach, priority level, etc. is evaluated at the field unit. Subsequently, the decision whether to grant or reject signal priority is made by the signal controller or the traffic control centre, depending on the system architecture. The second type of GPS-based detection allows for the determination of vehicle location, speed, and heading. This system requires the installation of radio transceivers on transit vehicles and radio receivers at intersections. The decision to grant signal priority to a transit vehicle is made based on the information (incl. location, speed, and heading information) which is sent from the transceiver to the receiver as the vehicle enters the intersection’s radio range.

Communication Systems

Communication systems are needed for interconnecting between components of signal priority systems. Depending on the system architecture (centralised or decentralised), requirements for communication systems can be different.

According to Intelligent Transportation Society of America (2005), basically, there are two types of communications as follows:

• The first type involves local communications between buses and signal controllers. Typically, these communications are integrated with detection systems.

• The second type is communications between local controllers and control centre. These communications can be deployed by means of physical connections (fibre optic or copper cable), or wireless technology. It should be noted that if physical connections have not been implemented yet, wireless communications would be more preferable in order to reduce infrastructure costs for communication systems.

Nowadays, mobile radio communications are important for numerous applications in public transport, particularly in Germany. These communications consist of analogue professional mobile radio (radiotelephony and radio data transmission), digital professional mobile radio, and Global System for Mobile Communications (GSM). Due to worthwhile cost-effective solutions, GSM technologies are emerging in the public transport sector with a number of applications to personnel management, Intermodal Transport Control System (ITCS), information systems, and security technologies. [BMVBW & VDV (2001)]