From: "Tony Sidaway" minorityreport@bluebottle.com
Road signs haven't switched because it would be very difficult and dangerous as well as politically explosive.
Actually, in the seventies, there was a widespread systematic effort to add distances in kilometers to road signs. I think it may have been an Interstate highway requirement. Many signs DID get changed, enough for them to become a reasonably familiar sight. Cars produced during that time period had both miles per hours and kilometers per hour marked on their speedometers, although odometers continued to read in miles.
I'm not completely sure what happened subsequently.
I do NOT remember any political "explosion" at all. Just apathy.
I believe that the requirement was dropped and that the dual-system signs were just gradually, in the natural course of sign replacement, replaced with signs giving distances only in miles.