MileCheckWhy do mile markers go up in one direction and down in the other?
A mile marker measures distance from one end of the route, usually the south or west end in that state. Drive away from that end and the numbers go up. Drive back toward it and they go down. The signs are the same. You are just reading them in the other order.
Both sides carry the same number
Mile 120 northbound and mile 120 southbound are across the median from each other. That is why a dispatcher asks for your direction as well as the number.
Which end is mile 0
On most routes the count starts at the south end for a north-south road and the west end for an east-west road. A road the state calls north-south can still run sideways for a while, so the numbers follow the route's official direction, not your compass.
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Questions, answered
If I'm going south, do the numbers go down?
On a north-south route, yes. They go down as you drive south and reach 0 near the south state line or the route's south end.
Does every state count from the south and west?
Most do. Some count by county or town instead. California's postmiles restart at each county line.