California is installing numbered exit signs based on mileage, finally coming in line with most other states.

The exit number will be based on the mileage from the road's origin. North-south highways will start at their most southern point, east-west from their most western point.
The new signs will be attached to existing signs when possible to reduce costs. It will take three years to put the 23,000 signs on 92 routes.
California never had to comply with a federal mandate for numbered exits, because it already measured road distances within counties by using white roadside posts, according to The Fresno (Calif.) Bee.