Improving road traffic safety is a worldwide political issue to be relieved urgently. In Japan, the road traffic accidents kill around ten thousand persons every year. A recent statistics suggests that about 75 % of accidents are caused mainly by some kind of human error. Accordingly, there is a great possibility that preventing human error in driving behavior might lead to reducing the number of traffic accidents. A lot of researchers and engineers expect that automation of driving tasks might prevent human error and enhance the road traffic safety drastically. Based on such expectations, Advanced cruise-assist Highway System (AHS) has been developed for the last decade in Japan. AHS is regarded as a sub system of Intelligent Transportation System (ITS). The hardware aspects of AHS are composed of sensors installed into roadway, communication devices connecting vehicle with roadway, and in-vehicle sensors and computers. AHS is expected to bring about well-coordinated relationship between vehicle, driver and roadway.