EVP — What Are the Technology Options?
There are many EVP technologies being employed today including lightbased, infrared-based, sound-based, and radio-based emitter/detector systems. As such, stakeholders must gather information and consider key operational features and interoperability requirements as they plan deployments and recommend EVP technology approaches. This section provides an introduction to key operational features that may be useful in assessing the available approaches.
Light and Infrared Systems
Light and infrared systems employ emitters that are normally mounted on the roof of the EV and are operated in conjunction with the emergency lights (Figure 2). The photograph on the left shows an early optical emitter mounted just under the windshield. The upper right photograph shows a factory-mounted emitter in front of the light bar. The lower right photograph shows a locally-installed emitter on the roof of a cab. The emitter system includes the light unit and a power supply that is located inside the vehicle.
Figure 2. Various Light-Based Emitters in Use Today
On the power unit, there is typically a control panel that allows selection of a high priority mode (used for EVP), and a low priority mode (used for transit signal priority). The control panel also includes a feature to assign unique codes to each vehicle operating on the system. The codes provide a record of which operator drove the vehicle, as well as protect against unauthorized use. Light- and infrared-based detectors are generally mounted on the signal arm. Mounting requirements include provisions for power and communications cables. Figure 3 shows both wire and mast arm mounted light-based detectors. Some jurisdictions install confirmation lights in conjunction with the detectors. This light provides feedback to the EV driver that the request for preemption has been received and that the request will be served according to the local preemption transition protocol.
Figure 3. Wire and Mast Arm Mounted Light-Based Detectors
Sound-Based Systems
Sound-based systems use the EV siren as the emitter. The waveform of the siren is loaded into the detection and processing equipment such that directional microphones mounted on the signal arm can detect sirens that meet the Federally mandated decibel level of 1,200 db. Once detected, the siren waveform is verified, a preemption request is generated by the phase selector and sent to the signal controller.15 Figure 4 shows sound-based detection equipment on a signal pole in Loudoun County, Virginia. The system pictured serves a regional hospital with EVP on two approaches.
Figure 4. Sound-Based Detection Equipment in Loudoun County, Virginia
Radio-Based Systems
Radio-based systems utilize a receiver with an omni-directional antenna to detect a digitally coded spread spectrum or narrow band radio transmission from an EV. In these systems, the direction of preemption is selected in the vehicle and direction-unique signal is transmitted to the intersection. Radio-based systems avoid the line-of-sight limitations associated with light- and infrared-based systems. Once a radio frequency pulse is detected and the proper direction of travel is determined, the preemption request is processed by the phase selector and the signal controller.
Table 2 summarizes the technical considerations of the various EVP options.
| Technology Consideration | Strobe Activated | Siren Activated | Radio Activated |
|---|---|---|---|
| Dedicated Vehicle Emitter Required | Yes | No | Yes |
| Susceptible to Electronic Noise Interference | No | No | Yes |
| Clear Line of Sight Required | Yes | No | No |
| Affected by Weather | Yes | No | No |
| Possible Preemption of Other Approaches | No | Yes | Yes |
15 Collura, J., and Willhaus, E.W. (June 2001). Traffic Signal Preemption and Priority: Technologies, Past Deployments, and System Requirements. Paper published in the conference proceedings of the ITS America 11th Annual Meeting, Miami Beach, Florida.
16 Ibid.