Highway Exit Numbers Carry Hidden Meaning - And Some States Still Resist the Standard

The Interstate Highway System marked its 70th anniversary this year, and while it spans nearly 49,000 miles, its design includes features meant to make navigation easier. Exit-numbering standards, for instance, tell drivers exactly where they are along the route. GPS may have made modern navigation routine, but those old-style markers can still be essential in certain situations.

The rules for exit numbering come from the Manual on Uniform Traffic Control Devices, the federal guide for road standards. It requires states to set the westernmost or southernmost point of an interstate at mile zero, then number exits based on distance from that point. An "Exit 4" would be four miles from the start, and multiple exits at the same mile get letter suffixes.

There are exceptions, though. The Federal Highway Administration didn't require exit numbering until 1970, and northeastern states were allowed to keep their older sequential numbering. New York State experimented with mileage-based exits in the 1970s but never adopted it, citing tight clusters of exits in urban areas that would create confusing strings of suffixes.

The 2009 Manual eventually mandated mileage-based numbering for all states, with loss of federal funding as the penalty for non-compliance, but no deadline was set. New York is still converting its exits corridor by corridor. Despite the slow pace, the mileage system is objectively better: adding or removing exits doesn't require renumbering the whole route, and drivers can report precise crash locations just by glancing at a sign.

The real obstacle is the cost of changing every sign.