Exit numbers are usually mile markers in disguise
On many American highways, an exit number tells you roughly where you are along the road—not how many exits you have passed. If an exit is near mile 42, it may be numbered 42. The next one could be 47 because the interchanges are five miles apart; nothing is missing from the sequence.
This system gives drivers a rough location at a glance. It also helps agencies describe where a crash, work zone, or road hazard is, even when it lies between exits. On a highway that follows mile markers, the number can point to the same stretch of road in either direction, while signs distinguish the direction or roadway.
Why the numbers can change—or seem to
Some older highways were numbered in order: Exit 1, Exit 2, Exit 3. As more roads adopted distance-based numbering, transportation agencies often had to choose between preserving familiar numbers and making exits correspond to their locations. A road converted to mile-based numbers may suddenly have larger numbers, and a new interchange can be assigned a number tied to its location rather than added to the end of a count.
Numbering conventions also vary by state and road. Some systems use kilometer markers, and some routes retain sequential numbering. A long highway may begin its numbering again where it enters a new state, so the same exit number can appear far apart on one continuous route.
The result is less a random code than a location system with local exceptions. Once you know that an exit number often approximates the mile along the highway, a jump from 12 to 18 becomes useful information: the next interchange is about six miles farther on.
