Basically GPS or 5th wheel are the only two super accurate choices. Some people have had luck with bicycle speedometers since you calibrate them to the rolling radius of the tires you have mounted. Because they list that exact set of responses on the FHWA website as being given in April of 2006. Initially I thought you were asserting that these people had contacted you directly, but I realized that you grabbed those responses off the interwebs. Others can read the exchange here: http://knowledge.fhwa.dot.gov/cops/...552797&Group=Travel Monitoring&tab=DISCUSSION Since the FHWA is not the correct department to ask, and neither the is the Washington state DOT, that is why they thought NHTSA could help. It sounds like the C&D article pretty much clears things up. The USDOT is the department that has all the little sub departments like the FHWA, NHTSA, FLHA, etc. but the ones at the top are the US DOT.