Recently, the dashboard illumination failed in the car I was driving. Trying to gauge my speed, at night, alone on a road with little side decoration was not fun.
If some sort of road marking - maybe the centre lines or the "cats eye" reflectors - were paced so that at the posted limits they went past at a constant rate, gauging the speed in similar situations would be easier. Perhaps, after time, we would develop a subconscious sense of timing for it.
(A quick calculation, by the way, shows that: if the rate is, say, 2s, and the driver is out by up to 0.5s, and the car speed is 60km/h, then the approximate error in the speed is 15km/h. At 100km/h, it's 25km/h.)-- Detly, Dec 23 2004random, halfbakery