In a rural area of Virginia, a collision countermeasure system installed on a two-way stop-controlled intersection reduced vehicle speeds by 2.4 mi/hr, and increased the average projected time to collision from 2.5 to 3.5 seconds.

1-4 May 2000
Aden,Virginia,United States

Summary Information

A collision countermeasure system (CCS) was designed for application at rural, unsignalized, two-way, stop-controlled intersections to improve safety. Operating on input from in-pavement loop detectors, the system automatically activates signs that graphically advise drivers of the presence and direction of approaching traffic. The system was installed on all approaches at the intersection of Aden Road and Fleetwood Drive in Aden, Virginia. A three-phase field study was conducted over 42 days to analyze the behavior of vehicles passing through the intersection before, immediately following (acclimation phase), and four months subsequent to system installation.

A literature review, conducted to develop an evaluation approach, revealed that 3.0 seconds are required to slow or maneuver a vehicle without coming to a complete stop and that it takes 4.6 seconds to completely stop a vehicle. Thus, it was concluded that advance warning times of 3.0 and 4.6 seconds were appropriate for measuring the system’s effectiveness. The following measures of effectiveness (MOEs) were developed for and utilized in the evaluation:
  • Sign Response Speed – speed at intermediate loop detectors located roughly 135 feet (41.2 m) beyond the sign and approximately 375 feet (114.3 m) in advance of the intersection.
  • Intersection Arrival Speed – speed at loop detectors located at the intersection.
  • First Speed Reduction – speed differences between advance loop detectors 950 feet (289.6 m) prior to intersection and intermediate loop detectors.
  • Second Speed Reduction – speed differences between intermediate and intersection loop detectors.
  • Overall Speed Reduction – speed differences between advance and intersection loop detectors.
  • Projected Time to Collision (PTC) – theoretical elapsed time to collide in the absence of a timely avoidance response.

A before-after analysis was performed on a 28-day, 63,000-vehicle data sample collected immediately before and four months following system installation. A targeted analysis of high-speed vehicles was also conducted, as these motorists pose the greatest accident threat. High-speed vehicles (the fastest 10th percentile and those exceeding 45 mph (72 kph)) arrived at the intersection in sufficiently close time proximity to represent real collision potential.

Although the observed sample size was approximately 97,000 vehicles, results were based on a targeted sample comprising 1,652 vehicles, which met specific intersection-arrival criteria. Lower approach speeds and safer projected-times-to-collision (PTCs) were observed in the presence of the system. While drivers did not slow down at the first loop detector, they did exhibit speed reductions approaching the intersection and lower speeds at the intersection. The before-after analysis revealed that average speed was reduced from 49.7 mi/h (80 km/h) in the "before" phase to 47.3 mi/h (76 km/h) in the "after" phase and that average PTC increased from 2.54 to 3.5 seconds.

For high-speed vehicles, average speed was reduced from 55.5 mi/h (89 km/h) in the "before" phase to 54.8 mi/h (88 km/h) in the "after" phase. Greater speed reductions observed in the "acclimation" phase were not sustained through the "after" phase. However, safer PTCs were exhibited. The average PTC of high-speed vehicles was greater than the critical 4.6-second accident-avoidance time requirement. The system was apparently effective at modifying behaviors of the intended drivers (i.e., those experiencing conflicts or accident potential with cross traffic). The elimination of all observed critically short PTCs for speed violators during the "after" phase is highly indicative of a safer intersection. The data and analysis support the conclusion that safer traffic operations resulted from installation and continued operation of the collision countermeasure system.

See also:

"Intersection Collision Warning System" (FHWA Publication No. FHWA-RD-99-103)

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Rural Stop-Sign Controlled Intersection Accident Countermeasure System Device Vehicle-Behavioral Evaluation

Author: Hanscom, Fred R.

Published By: Paper presented at the 10th ITS America Annual Meeting. Boston, Massachusetts

Source Date: 1-4 May 2000


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Goal Areas


Typical Deployment Locations

Rural Areas


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Benefit ID: 2000-00167