The timing error will come from two places: perspective error and human reaction time. Perspective error (where the top edge of the car appears to cross the line before the wheel fo the car actually does) is the ratio of the distance from the top edge of the car to the ground divided by the distance from the ground to the car. This makes the perspective error very small (perhaps 8'/800'). The way to minimize human timing error is to make the ratio of distance between markings large in comparison to the car's length, which is usually the case (1/4 to 1/2 mile is about what I see around here for markings). All in all the error from both factors is small and timings accurate.

-Zeke