The risk homeostasis conjecture states that the driver tries to keep a constant level of accident occurrence probability when he/she drives through an intersection with or without traffic lights. This constant can be regarded as a maximum acceptable probability of accident. The maximum value is reached, for instance, when the driver intends to minimize the traveling time by trading-off the accident avoidance. This paper first yields interval estimates of the homeostasis constant from the accident and road data. The constant turns out to have an order of the ten to the minus seven, a very small value. A safety system is considered for an intersection without traffic lights. A north-south driver of the first party is alarmed about the east-west vehicles of the second party approaching to the intersection. The first party driver is supposed to perform an avoidance action when an unsafe relation occurs between the north-south and the east-west vehicles. The east-west driver of the second party takes no avoidance action. The safety system succeeds in generating the alarm with a reliability value when required, and fails to generate the alarm with one minus the reliability. The north-south driver is assumed to be completely dependent on the safety system. A necessary condition for the safety system to become effective is clarified from the homeostasis constant and it requires the system reliability higher than three 9s, a significantly severe demand to the safety system currently available. An ordinary 90% reliability is far from sufficient and inadvertently increases the traffic accidents.
Abstract