A key issue facing localities in the U.S. is the challenge that rapid growth in populated areas places on the fire and rescue community. Providing safe and efficient public safety services is a primary obligation of local governments. Faced with tight budgets, officials must make decisions on how to provide appropriate levels of service in the face of increasing demand for services and increasing congestion levels.
The problem officials face is two-fold: Emergency vehicles (EVs) operating in higher congestion levels are at risk for involvement in crashes and are subject to delays in reaching the scene of a fire or trauma event. One means to offset the effects of congestion is the installation of emergency vehicle preemption (EVP) equipment at signalized intersections.
In justifying EVP deployment proposals, transportation and fire and rescue often state that the system will “lead to a reduction in EV crashes” and that reducing the potential for EV crashes is beneficial to the jurisdiction in several ways. First, EV crashes cause property damage, injuries and possibly fatalities that represent liability. Second, an EV crash puts the victims at the original response location in further jeopardy because the call will have to be filled by another unit, increasing the response time. Third, EV crashes often take apparatus out of service for extended periods of time forcing the remaining units to cover a larger area, potentially increasing response times to some zones within the jurisdiction and within neighboring jurisdictions that depend on mutual aid agreements.
One problem for the EVP advocates is that local EV crash records for a year or two will not generally be conclusive, and lessons taken from records for longer periods, such as five or 10 years, may lead to better trend information but are not generally conclusive because of the tremendous changes in traffic volumes, number of signalized intersections and the number of EV responses. Therefore, there is a need to develop an evaluation framework that examines EVP in terms of safety in such a way that unreasonably long evaluation periods are not required. The method proposed in this article uses a conflict point evaluation method derived from the study of EV-specific conflict points using methods typical in intersection safety analysis.
Conflicts of interest
The genesis of the methodology proposed includes conflict point analysis used in traditional design and operation of intersection traffic control devices. For the research that supports this article, traffic engineers studied EV intersection passage events using a combination of overhead video and ground-based observation. Findings revealed that there are three general classes of conflict point that are unique to EV operations as illustrated in Figure 1 and described below.
- Conflict with Concurrent Traffic Streams
Traffic flowing in the same direction as the EV was characterized by the potential for low-severity crashes (primarily rear-end and side-swipe) but the number of interactions with concurrent traffic could be very high depending on the traffic volume and whether the traffic stream was flowing and capable of moving to clear the path or stopped in multi-lane queues;
- Conflict with Perpendicular Traffic Streams
Traffic entering the intersection from an angle approach was characterized by the potential for high-severity crashes (primarily angle), but the number of interactions was relatively low depending on whether the side street was on a red or green signal and the presence of line-of-sight obstructions; and
- Conflict with Opposing Traffic Streams
Traffic proceeding in the opposite direction exhibited two sets of characteristics depending on whether the EV proceeded through the intersection or turned left across opposing traffic. When the EV proceeds through the intersection, crash severity potential is relatively low, but when the EV makes a left turn at the intersection there is a potential for very high-severity crashes (angle or head-on) as the EV turns across conflicting auto pathways unexpectedly.
Research supporting this article included a review of EV crashes at the national level using the Fatality Analysis Reporting System (FARS) and at the local level for the four counties that make up the northern Virginia region using state crash records. At the national level there are approximately 80 fatal crashes involving EVs per year. Of those, approximately 25% occur at signalized intersections. At the local level, within these four counties combined, there was an average of 15 EV crashes per year. Approximately 33% occurred at signalized intersections. The EV crash and conflict point study contributed to the development of “thumbnail” conflict point illustrations and severity indexes usable in safety audits. These are shown in Figures 2, 3, 4 and 5.
The potential for an EV crash to occur while crossing an intersection is related to the exposure of EVs to the type of conflict points that have histories of producing crashes and the density of the auto traffic. The severity of the potential crash is related to the geometry of the auto-EV interaction and the presence of intersection conditions that increase crash propensity. EVP seeks to reduce the number of active conflict points by bringing conflicting approaches and movements to a complete stop and reducing the density of the traffic on concurrent approaches by providing a green signal display ahead of EV arrival. In a typical “before” EVP case, the probability of encountering a particular conflict point is a function of the distribution of green time in the signal cycle and the related probability of an EV arrival during a conflicting signal phase. In a typical “after” case, with the conflicts controlled via a special EVP signal phase, the EV will arrive during the phase that produces the least number of conflicts and the least severe conflicts.
Using the conflict point analysis in an assessment or safety audit is simple, requiring typical intersection assessment data and minimum EV trip characteristic data as shown in Table 1.
More than just sirens
Using the conflict point descriptions and the associated severity indexes, stakeholders can assess an intersection quickly and can present the results in an objective format supported by both descriptive and quantitative information. For those considering resource apportionment decisions across several candidate intersections or corridors, the method can be used to generate information allowing policy makers to compare one intersection to another. It also can be used in a roll-up fashion to represent the relative merit of equipping a particular corridor over another.
In conclusion, EV crash reduction is a bona-fide objective of EVP deployments—EV-related crash histories illustrate the degree of the problem at the national and local levels.
Transportation and fire and rescue officials need tools to objectively indicate the need for EVP systems and this article offers a method which links crash potential to the EV-specific conflict points found at signalized intersections. Controlling these unique conflict points using EVP will decrease the number of and severity of the conflict points that pose a potential threat to EVs responding to emergencies within the communities they serve.
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