Exit Number - Distance-based Numbers

Distance-based Numbers

As more highways were built, the limitations of sequential numbering became clear, and states began to experiment with distance-based (mile- or kilometre-based) exit numbers. The first mile-based system known was implemented on the Garden State Parkway in New Jersey in the late 1950s. Michigan also implemented mile-based junction numbers on Interstate 94 in the 1960s. In this system, the number of miles from the beginning of the highway to the exit is used as the exit number. If two exits end up with the same number, the numbers are sometimes slightly modified; this is often impossible and exits are given sequential or directional suffixes, just as with sequential numbers.

An exit can be numbered by where the exit in the direction of increased mileage leaves the freeway, or by where the road that the exit serves crosses the freeway (which is occasionally ambiguous). The US Manual on Uniform Traffic Control Devices (MUTCD) recommends the latter. From this number, the integer exit number can be determined by rounding up, rounding down, or rounding to the nearest integer. Many jurisdictions prefer to avoid an exit 0. To this end, the numbers are either rounded up to get the exit number, or any exit that would get the number 0 is instead numbered 1. Examples of highways with an exit 0 are British Columbia Highway 1 on the B.C. mainland, Interstate 10 in El Paso, Texas along the New Mexico-Texas border, Interstate 70 in Wheeling, West Virginia along the West Virginia-Ohio border, Interstate 90 on the Montana side of the Idaho-Montana border, and Interstate 65 in two locations: at its terminus with Interstate 10 in Mobile, Alabama, and just north of the Ohio River in Jeffersonville, Indiana.

In areas that use the metric system, distance-based numbers are by kilometre rather than mile. A few highways, such as Delaware Route 1 and Interstate 19, have been renumbered from miles to kilometres, even in areas that typically use miles. I-19 has received funding for the distances to be changed back to miles.

Distance-based numbers have several advantages. They match the mileposts along the road; it is thus easy to calculate how far one has to go. Additionally, most new exits don't need letter suffixes, as in a sequential system. Two other advantages are outlined in this guide.

Suffixes are required when the same mile of highway contains multiple exits; an extreme example is along several different highways in Kansas City, Missouri, where there are 23 exits in the four-mile long "Alphabet Loop", designated 2A through 2Y (the I and O suffixes are skipped because of their resemblance to 1 and 0 respectively). However, for calculation purposes, suffixed exits in a distance-based system can be approximated by using the number without a suffix.

On the other hand, there are some disadvantages to changing from a sequential system. Businesses and motorists have to adapt to the changes, and it costs money to replace the signs (as well as for temporary "old exit" tabs to ease the transition). Due to a kilometre being 0.621 miles, the probability of multiple exits within the same kilometre is less than the probability of multiple exits within the same mile.