The Design of the Interstate Highway System
Overview
The Interstate Highway System is generally easy to navigate, but there are underlying standards that will help you navigate the Interstate System more easily in the future. Specifically, you should know the standards of route and exit numbering, signs, and exceptions to the rules.
Road Layout
Interstate Highways have at least four lanes, with two lanes going in either direction. There is typically a grade (slope) or barrier in the middle of the highway. There are assorted other federal standards, too, such as:
- Lanes must be 12 feet wide.
- Grades/slopes must be within federal regulations.
- Signs must meet the Federal code.
Major Routes
Major routes are Interstate highways with one or two digit numbers. Typically, major routes connect large urban areas in paths that are mostly straight. These highways tends to be heavily traveled and have lots of traffic.
Major routes are numbered based on their position and their direction, using the following criteria:
- Routes that go south - north have odd-numbered endings.
- Routes that go west - east have even numbered ending.
- The further west and south the route is, the lower the route number will be.
- The further east and north the route is, the higher the route number will be.
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| The major routes of California. The route farthest west is I-5, and the route farthest south is I-8. |
So, based on the criteria above, routes 1 and 2 would be the furthest southwest and routes 98 and 99 would be the furthest northeast. You can use the major route highway numbers to your advantage. If you know the number, you can tell if you're headed on a road heading north or south and roughly how far east or west you are.
Note that the Interstate Highway System numbering scheme is the opposite of the US Highway system. In the US Highway system, the lower numbers were further northeast. The Interstate Highway System uses a different standard because the Federal government based its numbering on California's work on its own state-wide highway system.
Bypass Routes
Routes with three digit numbers are called bypass routes. These routes are numbered based on the following criteria:
- The major route the bypass is off provides the last two digits of the bypass route number.
- Spurs off major routes into cities start with odd numbers.
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| Route I-90 in Massachusetts with the spur route I-190 heading off into the north central portion of the state. |
- Routes that circle around urban areas off a major route (circumferential routes) start with even numbers.
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| Route I-40 in Tennessee with the (proposed) circumferential route I-840 around Nashville. |
Bypass route numbers are similar to major route numbers in that the lower numbers are further south or west. As the major route goes further north or east, it could have another bypass route with the next highest spur or circumferential route. Also, the naming system does not carry over from state to state, so there is a 190 in both Massachusetts and New York.
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| The bypass routes of California. Notice how they're centered on the dense urban areas of San Francisco and Los Angeles. |
Here's an example of how it works:
I-10 is the furthest south transcontinental Interstate Highway. It starts in Santa Monica, CA and ends in Jacksonville, FL. In California, there's a I-110, which is a spur into Los Angeles. Going further east to Louisiana, there's another 110, which is a spur into Northern Baton Rouge. Still further east is 210, a circumferential route around Lake Charles, and 310, a spur into Kenner.
Exceptions
Of course, like any useful standard, there are exceptions to the rules. Sometimes these exceptions are due to political handiwork, other times the Federal government genuinely ran out of available numbers, or left some numbers open for future expansion. Here are some exceptions to the Interstate Highway System numbering conventions:
- The route furthest west is I-5 (there is no I-1 or I-3.)
- The route furthest south is I-4, which crosses the peninsula of Florida (there is no I-2.)
- The furthest north is I-96 (there is no I-98.)
- I-99 is in Pennsylvania, but it's actually between I-79 and I-81. I-97 is truly the farthest east.
- Hawaii has its own set of "Interstate" highways: H1, H2, and H3. Alaska has no highways recognized as Interstates, but several of their highways are federally funded (like Interstates.)
- I-238 in California is an orphan - there is no I-38. I-238 was originally CA-238, but they never changed the number.
Exits
Exits follow the same general convention as route numbers: The farther southwest the exit is, the lower the exit number is. There are two ways exits are numbered, and it is usually decided on a per-state basis.
- Consecutive. Exits are numbered consecutively, i.e., 1, 2, 3, etc. These exit numbers are easy to understand. However, if a new exit is created, exits must be renumbered or letters could be introduced to the number, such as 2A, 2B, etc.
- Milepost. In this numbering scheme, exits are numbered based on the closest milepost, rounding down. So if you had an exit between mile 3 and 4, it would be exit 3 (note that the beginning milepost in a state is zero.) This numbering scheme is useful because you can easily tell how far you are from your destination.
For More Information
For more information about how the highway design, try the following:
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