Evaluation of the Prince William County Collision Countermeasure System

About the report:

The Collision Countermeasure System (CCS) is an ITS application intended to reduce side-impact accident potential at rural, limited sight-distance intersections. It consists of activated warning signs and pavement loop detectors designed to enhance driver awareness of cross traffic. This field evaluation, comprising a four-phase observational effort (before, acclimation, 4-month after, and 1-year after studies) assessed novelty and longer-term CCS effects. Results reported herein are based on a 48-day, 109,000-vehicle data sample. In order to address CCS accident reduction potential, the study targeted 2,242 high-speed vehicles arriving at the intersection in close time proximity to cross traffic. This study also assessed the potential “familiarization” effect, i.e., driver response to the device in its non-activated condition and conducted a benefit/cost analysis. Vehicle behavioral measures of effectiveness (MOEs) were derived from CCS accident-avoidance objectives, i.e., specifically addressing intersection arrivals in close proximity to cross traffic. Applied MOEs were (1) drivers’ CCS speed responses in the presence of cross traffic; (2) intersection approach speed reductions; and (3) projected times-to-collision (PTCs), i.e., the elapsed time to which an approaching vehicle would collide with a vehicle in its path in the absence of a timely avoidance response. Human factors (e.g., driver perception-reaction time) accident-avoidance requirements determined the critical PTC values that were used in the analysis. The vehicle-behavioral field evaluation produced the following results: (1) lower intersection-approach speeds were observed following installation and 1-year operation of the CCS; (2) longer PTCs, indicating a safer condition in the presence of cross traffic, were observed following CCS installation; and (3) sampled high-speed vehicles, i.e., exceeding 72 km/h and 88 km/h, exhibited initial novelty-effect CCS speed reductions, which were not generally sustained during extended CCS operation. However, this sample nevertheless demonstrated reduced accident potential, i.e., longer PTCs, during extended CCS operation. A “driver familiarization” study demonstrated that vehicles did not increase speeds during periods when the CCS was not activated. A cost-effectiveness analysis produced a positive result. The average annual (1993 through 1997) accident property damage and injury cost for the studied intersection far exceeded the estimated annual CCS cost, i.e., sum of CCS capital recovery, manual monitoring, and operation/maintenance costs. In simplest terms, if the CCS prevents one side-impact accident per year, the device is cost-effective.

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