For designing crash avoidance systems it is necessary to know the dynamic conditions that characterize the accidents. This paper examines the dynamic conditions that set apart safe from unsafe lane changes. They are determined on the basis of the relative distances and velocities between the vehicles at the time the lane change is initiated, and upon consideration of the geometry of the lane change path. The analysis provides means to quantify the significance of the errors of the measurements and estimations that the countermeasure system must carry out to achieve its goal. The discussion includes consideration of system's latency and reaction times of driver and vehicle, to compare them with the time between lane change initiation and the moment when the crash would occur. The paper considers possible different levels of capability for lane change countermeasure systems, and how they could be verified by tests without crash risk.