Typically timing runs are done in two runs and the two runs averaged. This way tail winds and slight hills can be evened out.
So make your first run (mark your time) and then turn around and make your second run in the opposite direction from the first. Add these times together and divide by two to get your average.
I think this might help with the people who don't realize that they are going down a steep hill with 35mph tail winds. ha ha.