It's an estimate of course, but the way we do it, is we always have a car or a motorcycle follow. The motorcycle is better, which is what was used in this case. You mark off a mile and makesure your speedometers are very close, and then you have that person used markers to make sure they are passing them at the same time you are. Like a sign...
I rode by the sign, and then the guy behind me counts how many seconds it takes for me to get from the sign to the next marker.... when he counts the from the sign to the marker for himself, we have to see if it took him the same time to pass between the points as me. If not, then we have to do it over again on the next sign we find....
Pretty simply and fairly accurate if you don't have a radar gun, which I do have... the kind that you use to hit baseballs with, which does a good job here to. Its hard to get someone to time me with the gun though, because I have to get someone to come out on a long open road and stand there to time me... not exactly fun!