A rear-end collision warning system for heavy-duty trucks using laser radar was put on the market in Japan in November 1989 with the aim of reducing highway traffic accidents. The basic concept of this system was formulated from the fact that about 70% of highway accidents involving trucks are caused by careless driving, although truck drivers are generally more skillful than ordinary drivers. Therefore, this system has been developed as a support tool for improving driving safety. Fleet tests were conducted by users for the purpose of determining suitable danger criteria matching human senses. The obstacle detection area and warning logic were optimized through these tests. Questionnaire were also given to 104 drivers to investigate the market response to the system. The responses included several examples of situations where an effective warning was provided under potentially dangerous conditions. The effectiveness of the system in preventing accidents was also confirmed statistically.