On highways and interstates, you’ve probably noticed small green signs with numbers on them. Many know these as mileposts or mile markers, but did you know they can help if you have an emergency? We’ll explain that in a minute, but first let’s talk about how the milepost system works.

Interstate mileposts

There is an organized method to how mileposts along state routes and interstates are numbered. Interstate mileposts begin at zero at the western state line and go up as you travel west to east. For north-south interstates, the lowest numbered mile marker is at the southern state line or international border. Knowing this can help you determine how far you are from a border. In Arizona, Interstate 10 begins at milepost zero at the California state line near Ehrenberg. The interstate travels through the Deck Park Tunnel in the heart of Phoenix at milepost 145 and continues on to Tucson, where the milepost number hits 255. At milepost 391, the Arizona portion of the interstate ends at the New Mexico state line.

State route mile markers

The numbering of Arizona state routes is similar. Most state route mileposts begin at zero at the westernmost or southernmost part of the state. Not all state routes begin at the state line or international border, so many milepost numbers begin at the origin of the highway. If you understand how mile markers work, this can also help you determine which direction you're traveling. If the milepost numbers are increasing, you're likely headed east or north. As with most things, there are definitely exceptions to the rule! One is the use of kilometers on Interstate 19. An Arizona Highways blog(link is external) explains how this interstate is unique.

Highway safety

Now, back to how these little green signs can help you in an emergency. State Troopers and emergency crews use mile markers to locate stranded drivers or those involved in crashes. If you ever have to call 911 on a state highway or interstate and you’ve paid attention to the mileposts, emergency crews will be able to find you faster. Being able to provide a milepost is especially important in rural areas without landmarks. Exit numbers also typically correlate to milepost numbers.

Our website has a map of Arizona’s mileposts(link is external) in case you want to check out which mile marker is closest to you.