Why 178? A guide to interstate exit numbering

Have you ever wondered why 411th Avenue in Tonopah is Exit 95 off Interstate 10? Or why if you are turning on to Dragoon Road in southeastern Arizona you take Exit 318?

Or how about using Exit 67 to get to Dateland from Interstate 8, or Exit 322 on Interstate 17 to get to Munds Park?

The answer is deceptively simple: It's all about mileposts.

So, quick background: Mileposts for an interstate start at "0" at either the western or southern state line, and increase as you head in the opposite direction, according to the Federal Highway Administration's page on its website about the interstate system. (The exception in Arizona being I-17 doesn't start at "0" because it took over its alignment and mileposts from the original State Route 69, but that's another story.)

As you head either east or north, the first exit you come to is numbered by the milepost immediately west or south of it. So if an exit is at milepost 8.7, it would be signed as Exit 8. If the next is at milepost 15.2, it will be Exit 15. And so on and so forth. And in case there are multiple exits at the same junction, like in the photo here, you use sequential letters to differentiate the ramps.

Sounds simple, right?

Except that's not always true countrywide. Some states, mostly in the East, don't use a milepost numbering system but rather a consecutive numbering system for their exits. So, coming from the west or south the first exit will always be Exit 1 and the next – no matter how far away – would be Exit 2.

However, the milepost system (or properly the reference location system) is the Federal Highway Administration's preferred way of numbering exits. In the 2009 revision of the Manual of Unified Traffic Control Devices (the federal guidelines for all things highways such as signs, pavement markers, signals, etc.), language was added saying, "Interchange exit numbering shall use the reference location sign exit numbering method. The consecutive exit numbering method shall not be used."

In Section 2E.31, paragraphs 4 and 5, the manual goes onto say why FHWA prefers this over the consecutive method: "Reference location sign exit numbering assists road users in determining their destination distances and travel mileage, and assists highway agencies because the exit numbering sequence does not have to be changed if new interchanges are added to a route"