Abstract: An apparatus is provided to facilitate the accurate measurement of changes in velocity of a vehicle. The apparatus (10) comprises a firing box (14) and a remote control box (16) for controlling a sequence of operations. A plurality of individually actuated solenoids (46) mounted within the firing box (14) actuate the firing of a plurality of marker cartridges (104) from a magazine (94) mounted to the firing box (14). The firing box (14) is mounted to the exterior of a vehicle (12). When a brake sensor (144) signals the actuation of a brake in the vehicle, the signal actuates the sequential firing of the cartridges (104) in equally spaced time increments to leave indicia on the surface of the ground over which the vehicle moves.