Safety and Crash Risks with Vehicle Strings


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National Surface Transportation Safety Center for Excellence


In the course of many previous studies on nighttime safety, the research team came to believe that there is a potential safety benefit of cars traveling in stabilized strings during free-flow conditions. Recent technical advances in vehicle-to-vehicle communications open the opportunity for vehicle safety systems to share information about driving conditions that can be used to improve safety. This project seeks to understand the safety implications of vehicles operating in strings to inform how effective cooperative driving might be at improving driving safety. In terms of driver behavior, the study revealed the following by comparing vehicles with and without a leading vehicle.

For driver behavioral differences at freeway ramp locations, overall, the study showed that drivers following other vehicles tended to travel at lower speeds than those who did not follow another vehicle. In addition to the lower speeds, however, vehicles with a leading vehicle frequently showed higher acceleration activity, seemingly suggesting that they were adjusting speed relative to the leading vehicles. In addition, the study also revealed that, during daytime, significant driver behavioral differences between vehicles with leading vehicles and those without leading vehicles were more evident at entrance ramp locations. Significant differences during nighttime tended to be more evident at both entrance and exit ramp locations but only for the analysis segments that are farther away from the ramp junction.

For driver behavior differences at intersections, similarly, the intersection analysis showed that drivers following other vehicles tended to travel slower, but with higher acceleration variance and jerk. These behaviors are likely due to the drivers needing to adjust speed more when following other vehicles. Drivers might also switch lanes and/or turn faster at intersections due to greater confidence about the roadway condition and/or fewer interactions with other vehicles when traveling in strings.



Vehicle strings, Leading vehicle, Following vehicle, Vehicle-to-vehicle communication, Cooperative driving