You know, I suspected someone would take issue with my assertion, and I agree that Clarkson quotes don't count, so here you are;

http://www.sha.state.md.us/safety/oots/trafficsignalsandlaws/speedlimits2.asp

http://www.tfhrc.gov/safety/speed/speed.htm

*from site campaigning for higher speed limits* http://www.safespeed.org.uk/speedlimits.html

*please note my previous comments apply mainly to motorways (and similar fast dual-carriageways) and are not applicable to urban streets*