City mileage signs

[Quillz]

What is the point being used to determine how far away any given city is from the highway that travels through it?

For example, most mileage signs for Los Angeles that can be found on the 101 are based exactly on the 101's terminus at the East LA Interchange. So around Exit 90, for example, a sign for Los Angeles will say "90 miles." But what about other mileages? Is it based on the city limits, the center point relative to the highway?

[Rothman]

Wasn't there a somewhat recent thread on this?

[epzik8]

Center points or "zero points".