6.16.1 Emergency Vehicle Pre-emption Schemes
Emergency vehicle pre-emption schemes (EVP) are of several types (Austroads 2007b):
- localised schemes at or in the neighbourhood of EV stations
- route pre-emption schemes
- active detection at signalised intersections
- GPS tracking.
The phasing and/or timing of traffic signals located close to an EV station are pre-empted on demand, either from a push-button at the station or a controller input activated by an in‑vehicle transmitter, to provide the EV controlled, priority access to the intersection. Further details of this form of EVP can be found in Part 10 of the Guide to Traffic Management (Austroads 2019d).
Route pre-emption utilises signal coordination systems to facilitate progress of an EV along a pre‑defined route. It is based on the establishment of key routes from the EV fire station and individual route pre-emption plans. Figure 6.30 shows an Adelaide example (Baskerville 2006). The route pre-emption plan is designated by a four digit number such as 3103. This number identifies both the station (first two digits) and the route pre-emption plan (second two digits). Upon request for a call-out, a crew member phones the traffic management centre through a special phone connection and the centre activates the desired route pre-emption plan which the emergency vehicle will use to get to its destination. Once activated the route pre-emption plan alters the signal timings of each intersection along the route using SCATS to favour the progression of the emergency vehicle along its route.
Because the system is based on a pre-determined route pre-emption plan, it can be susceptible to unforeseen circumstances along its route such as traffic congestion. This problem can be overcome through the EV staying in contact with the traffic management centre so that the pre‑emption plan can be restarted if not coinciding with the emergency vehicle’s progression. This could overcome the situation of an emergency vehicle falling completely out of synchronisation with the system and obtaining negative priority, i.e. stopping at many red lights (Austroads 2007b).
Route pre-emption schemes of this type are more suited to fire service vehicles than to ambulances because the former have slower acceleration from a stationary position and thus benefit more from good signal progression, and they generally depart from a fixed base as opposed to an on-road callout which is common for both ambulance and police services.
Source: Baskerville (2006).
There are three general methods used in EVP systems to detect the arrival of an emergency vehicle (Federal Highway Administration 2006a):
- Strobe activated – an infrared or light (optical) frequency transmitter is installed in the emergency vehicle and a roadside receiver is installed at an intersection to pick up the arrival of the transmitted signal.
- Siren activated – the noise frequency of a siren is employed to activate sensors at an intersection to identify the arrival of an emergency vehicle and activate signal pre-emption.
- Radio activated – a radio frequency signal is transmitted from a vehicle-mounted transmitter and received by a roadside receiver; a radio activated system is more susceptible to electronic noise when compared with the strobe activated system, but does not require line‑of-sight reception.
Active detection is not common in Australia or New Zealand, but a Melbourne trial of a strobe based system has given encouraging results in terms of reduction in response times and improved safety (Bean & Studwick 2006).
A GPS-based system utilises a GPS receiver which is installed both at an intersection and within the emergency vehicle. The system can ascertain the position of the emergency vehicle relative to a signalised intersection and can also determine the vehicle’s speed. This enables the system to predict the estimated time of arrival of the emergency vehicle at the intersection (Austroads 2007b).
A GPS-based pre-emption concept using the STREAMS traffic management system is described in Transmax (n.d.). If the emergency vehicle’s route is known, STREAMS ensures that the emergency vehicle receives a green phase upon arrival at an intersection and also alters the signal timings in advance. This allows vehicles at the signalised intersection to move out of the way more easily, hence decreasing the emergency vehicle’s travel time. If the route is unknown, the system can predict possible routes and implement a pre-emption strategy on each route, eliminating route choices and returning signal timings to normal, based on the progression of the emergency vehicle. In both cases, signal timings return to normal once the emergency vehicle has passed through the intersection (Austroads 2007b, Transmax n.d.).