Highway exit numbers typically start from one as the highway enters a new state. The exit number increases by one unit per mile until the highway crosses the state line.
Two exit numbering methods in the United States are sequential numbering and distance-based numbering. The sequential numbering method numbers the exits sequentially. When adding a new exit between two existing exits, the new exit has a suffix in addition to a number, such as Exit 21A on the New York State Thruway. Some new exits in Florida have the suffix C.
The more common exit numbering method is distance-based numbering. Most exit numbers start at one and the number increases by miles traveled. Some highways, such as the Interstate 70 in Wheeling, WV, start with exit 0.