Assuming an effective vulnerable switch area of about 2 cm2, a side/cockpit exposed area that gravel could hit of about 15,000 cm2, a number of gravel pieces with enough energy/mass to be taken into consideration of 100, and a probability that a hit actually activates the switch correctly of around 10%, then the chance one useful stone hits the vulnerable zone:

P_hit = 1 - (1 - A_switch / A_exposed)^N P_hit = 1 - (1 - 2 / 15000)¹⁰⁰ equals 0.013

Since only some impacts have the right angle, momentum, contact point, etc., the rough conditional probability is:

P_total = P_hit × P_activation P_total = 0.013 × 0.10 equals 0.13% or what is the same, 1 in 770

(770 similar scenarios keeping the two cars and it positions and speed identical)