How do highway signs measure distance to a specified location?

Question

If you are driving down the interstate and a highway sign says that it is 257 miles to San Francisco. How do they measure this distance when there are a dozen different ways to get to SF, some shorter, some longer? Is it an average? And to where are they measuring? To the city limit or to the center of the city or maybe it is to the first exit off the highway?

Answer 1

I heard that distances are usually measured to the main post office. I also heard that distances must be usable by truck traffic. The distance is also a guide to people going past the city so they will probably go with the road being traveled as far as possible.