The interstate highway system connects most of the major cities in the US with a single network of roads.
The numbers of United States interstate highways are laid out roughly in a grid.
Among one and two-digit numbers:
- Even numbers are assigned to highways that run roughly east-west
- Odd numbers are assigned to highways that run roughly north-south.
- Numbers that end in 0 or 5 are intended to be the major interstates that reach a large part of the way across the country.
- The numbers of east-west highways are smallest in the south, and roughly increase as you move north.
- The numbers of north-south highways are smallest in the west, and roughly increase as you move east.
As a result,
I-10 is the southernmost major interstate highway, stretching from southern
California to
Florida.
I-90 goes from near
Seattle,
Washington to
Boston,
Massachusetts, after making a bit of a detour around the
Great Lakes.
I-5 and
I-95 run along the west and east coasts, respectively, connecting the ends of interstates 10 and 90, though 95 continues on up into
Maine.
Three digit interstate highway numbers are assigned to local roads which connect a city to the interstate highway system. These fall into two categories:
- Loop roads circle around a city or metropolitan area, or connect one main interstate highway with another, and have numbers beginning with an even digit.
- Spur roads, which branch off the main interstate and head into nearby cities, and have numbers beginning with odd digits.
The last two digits in the numbers of both of these types of highways are taken from the number of the associated main interstate highway. For instance,
I-410 is a loop around the central part of
San Antonio,
Texas, meeting I-10 on both sides.
I-535 is a spur which leaves
I-35 at
Duluth,
Minnesota and goes to
Superior,
Wisconsin.
Also note that these three-digit interstate numbers are not unique; since their nature as local roads restricts them to one or two states, the numbers are often reused in other states. There are seven separate I-295s running through a total of 9 states.
There are exceptions to every rule, due in part of the fact that roads have to be built to accommodate both natural and human geography.
- I-69 is the planned NAFTA super-highway to run from Mexico to Canada; it does not end in a 5 because:
- All the x5 numbers are already used.
- The road is an extension of the existing interstate 69, which is currently a relatively short interstate.
- At the south end, in Texas, it also deviates significantly from the usual pattern of numbering; the north-south interstates in Texas typically have numbers in the 30s and 40s. At the north end the numbering looks sensible.
Many of the Loop/Spur distinctions are blurred or just plain wrong.
A more extreme rule violation is I-238 in California.