A collision risk criterion based on imaginary brakes

This study proposes a risk criterion for rear-end collision warning systems. A warning system evaluates a driver´s risk and gives an alert when the risk exceeds a threshold, where the risk criterion employed is crucial. Our previous studies have shown the effectiveness of the 2nd-order time-to-collision (TTC) that assumes a constant acceleration motion for vehicles and calculate the time until a collision. However, the criterion considers such a case to be safe that a driver follows the preceding vehicle at a constant distance, no matter how small it is. Obviously, this does not agree to a driver´s feeling of risks. To take this issue into account, we make an assumption that a driver always expects the preceding vehicle´s sudden brake and propose the brake-assumed 2nd-order TTC, which is the 2nd-order TTC when the preceding vehicle decelerates with fixed brake strength. This idea is simple but difficult to determine the parameters, especially the brake strength. We propose two methods for estimating a driver´s imaginary deceleration. One is called the CV minimizer and the other is called the transferred deceleration. In order to confirm the effectiveness of the methods, we apply the methods to the driving data collected originally under unconstrained conditions. In the experiments they show almost the same results where the criterion explains the brake use as well as the throttle use.