The information presented here is designed to help you locate rest areas along Interstate highways. Understanding how the information is presented, and learning the difference between a mile marker and exit number, will aid you in using this site.
Mile Markers
Mile markers, or mileposts as they are also known, are the vertical green signs on the edge of highways placed at one-mile intervals. Mile marker numbering begins at the most southerly or westerly point in a state. For example, if you enter Colorado from New Mexico, mile markers will increase as you travel north through Colorado. Likewise if you were to enter Colorado from Utah, mile markers would increase as you travel east through Colorado. California is the only state that does not use mile markers. Instead they use a Post Mile system with numbering beginning and ending at county lines.
Exit Numbers
Interstate exit numbers are determined by one of two methods. The first, and most widely used, is based on the mile marker system. Using this method, the first exit number on an Interstate as you travel south to north or west to east is determined by its distance from the state line. For example, if an exit is located between mile markers 4 and 5, it is numbered as Exit 4. The next exit, if located at mile marker 8.7, would be numbered as Exit 8. Thus you would know that you must travel approximately 4 miles to reach the next exit. Using this method of exit numbering helps to determine the location and distance to a desired exit.
The second method of numbering Interstate exits is the consecutive numbering system, which means Interstate exit numbers begin at the most southerly or westerly point and increase consecutively as you travel north or east. Using this method, the first exit on an Interstate as you travel south to north or west to east is Exit 1. Each exit thereafter increases consecutively as Exit 2, Exit 3 and so on. Few states use this method of numbering Interstate exits.
California’s Numbering System
As mentioned above, California does not use mile markers nor does it indicate exits with a number. This is changing, however. In January of 2002, California began erecting signs displaying exit numbers based on the mile marker system. The exit numbers and mile marker numbers used on this web site are based on the proposed numbers that California has assigned.
The Interstate System
Watch this interesting 8-minute video to learn about the Interstate highway system.
Disregard my previous post. We are northbound but on an even (east/west) Interstate hwy.
This is completely opposite.
We just entered Washington from Oregon and the numbers were at 131 and are decreasing as we travel north.
You must be referring to I-82. This interstate is an east-west interstate despite its north-westerly trajectory, therefore it is sensible that the exits decline as you travel west on the interstate toward I-90/Seattle. I-82 was originally proposed as a part of I-80N which is why this interstate remains an east-west even numbered interstate.