I knew a guy that went back to southern California to visit and was doing 55 when that was the speed limit. Cars were zinging around him like he was sitting still. A CHP pulled up along side of him and sternly motioned for him to speed up. The basic speed law says to go with the flow of traffic.
I have often been amazed/taken back by this when I see traffic going 70-75 mph. It seems to me that speed in some situations is just a matter of ones get go.