How Mile Markers Work

If you’ve read our previous history on Mile markers, you’ll know that mile markers themselves come from the ancient roman use of milestones—but how did they get adopted by the interstate highway system? First, you’d have to know how our highways and interstates are numbered. Routes and interstates are laid out in a grid network fashion. Routes that run east and west are running perpendicular, (across), routes the run north and south.

North and south routes follow the same grid. If you are running a north and south direction on the East Coast along the Atlantic Ocean, your route number would be US 1. Running north and south along the West Coat beside the Pacific Ocean, your route number would be US 101.

For Interstates, the numbering is opposite from Routes, however it uses the same grid method. For Interstates, ones that run across, east and west, are even numbers while ones that run north and south are odd. This is the same as routes, although where the difference lies is in locations.

Originally, these routes were identified with different names, such as “The Pilgrims Way, Watling Street, and The Via Appia”—but when traffic increased, reference points were used to define specific locations. These reference points were later referred to with letters—which also proved to not be the most efficient way to refer to different locations. Mile markers were later installed to follow the interstate grid pattern within each state, started from west to east and south to north—indicating how many miles one had traveled within a state and how many miles one had left in that state before reaching the next one.

Mile markers can tell you what direction you are going. For example Interstate I-10 practically runs coast to coast, from Jacksonville Florida to Santa Monica California. It is 2460 miles across. However the mile markers will not count up from 1 to 2460 heading east bound from California, instead they’ll start over at the state border.

There are a variety of mile markers. On the interstates, they are basic green vertical signs that say “Mile” above a number.

On most highways, mile markers coordinate with the exit numbers. If you know you're headed for Exit 57 and you just passed mile marker 47, you now know you've got about 10 miles to go until your exit. In this way, mile markers can help you keep track of where you are and how far away you are from certain exits.

California is the only state that doesn’t mile markers—instead they use a “postmile” system.” The postmile markers indicate the distance a route travels through individual counties, as opposed to mile markers that indicate the distance travelled through a state. Multiple other states, including Nevada, Ohio, and Tennessee, use similar county-based mile markers on non-interstates, but use standard mileposts on interstate routes. As a former Ohio resident, this makes sense remembering how county roads were often the directional guides to get places in more rural towns outside of Toledo.

Whether you’re on a highway or an Interstate, looking at a postmile sign or mile marker, think of us, making all of the signs that help you get to your destination! If your state is in need of any, hit us up for a free estimate.