[TexasSP]

So many factors affect how your eyes gauge speed, you can never use them realistically as to determine it. This was something I had a conversation about with my brother (who is a cop) and some of his cop friends several years back. They were discussing how impossible it was to accurately tell a vehicles speed just watching and how most of the time there guesses were way off when compared to the radar readings. Things they said affect it greatly is other traffic on the road, trees and height of them, road width, weather (clouds, sun , etc), time of day, and noise level around and from the vehicle.