Elucidation of brake discontinuation mechanism in rear-end collision

In 2018, rear-end collisions accounted for 35％ of traffic accidents between vehicles in Japan and they accounted for the greatest proportion. Moreover, multiple collisions accounted for 13％ of rear-end collisions in Japan from 2007 to 2012. In a rear-end collision, sometimes a driver’s foot in a struck vehicle comes off the brake pedal and this may cause multiple collisions. A post-crash safety system is one of the technologies used to ensure post-crash safety. It assists the driver to stop the vehicle by operating an automatic brake after rear-end collision, so that prevent multiple collisions can be prevented. Practical use of this system has already begun in Europe. Based on previous research, a post-crash safety system has the potential to avoid multiple collisions; however, the system has not yet found practical use in Japan. The purpose of this study is to elucidate the mechanism of brake discontinuation and the collision condition that causes brake discontinuation in a rear-end collision. First, the behavior of a driver’s lower leg when a rear-end collision occurred was analyzed based on physical crash tests using a K-car. In addition, the relation between the brake operation of a driver and the speed of a struck vehicle when a rear-end collision occurred was analyzed based on physical tests and a drive recorder database. The results confirmed that the backward movement of a driver's ankle after a rear-end collision, which depends on the speed of a struck vehicle, affects the brake operation.Second, a formula was derived to estimate the striking speed when the brake discontinuation probability of a struck vehicle after a rear-end collision was 99.9％ by logistic regression analysis. Based on this formula, the striking speed that caused brake discontinuation in the event of a rear-end collision between a normal passenger car and a K-car was identified as 10.2km/h. Also, the striking speed that caused brake discontinuation in the event of a rear-end collision between cars with the same mass was identified as 15.3km/h. Moreover, the frequency of rear-end collisions at both these striking speeds is high.Therefore, a post-crash safety system operating at these speeds is an effective measure to prevent multiple collisions in Japan.