They have these light pole-mounted solar-powered radar boxes around my city here in So-Cal that will display your speed. The other day I was riding on a short downhill towards one of these boxes, with a 40mph posted speed limit.

I sprinted down hill with everything I had and finally tripped the device when I was practically on top of it. My speed? 45mph! (Corroborated by my computer) Woohoo! I wish I could snap a picture and sprint at the same time!