A drag racing speed trap covers the final 66 feet before the finish line. It reports the vehicle’s average speed over that closing section of the run.
Timing systems use separate measurements for reaction time, elapsed time, and speed. Elapsed time runs from the car’s launch at the starting line to its crossing of the finish. The speed trap is positioned near the end, where the car is usually at or near its highest speed, so the figure is commonly called trap speed.
Trap speed is not the same as the car’s instantaneous peak speed. Because it averages motion across the final 66 feet, acceleration, gearing, traction, and aerodynamic drag all affect the result. Two cars with similar elapsed times can therefore show different trap speeds.
The standard drag-racing distance is traditionally 1,320 feet, or one quarter-mile, although many modern professional Top Fuel and Funny Car runs use 1,000 feet for safety reasons. The speed-trap measurement remains a useful diagnostic regardless of the overall race distance.