Should you find yourself as a motorist in the Continental United States, with a destination somewhere else in the Continental United States, America's Interstate Highways are the best way for you to drive around while spending a minimum of time and money. Before you get on an interstate, there are a few rules you should know, so you can get approximately where you're headed without a map.
The majority of Interstates occupy land in more than one state, giving them their 'inter-' state nature. If they remained entirely within a single state, they would be 'intra-' states. However, this is not a hard and fast rule: some highways in single states received funding under the interstate programme, and thus have an interstate designation.
The majority of interstates are numbered with regard to their direction. Routes with a general north - south direction are oddly-numbered, while east - west routes have even numbers. Numbers are lowest in the south and west (eg Interstate 4 in Florida, and Interstate 5 through California).
Spur routes, that is, routes which are attached to interstates without necessarily being fully fledged interstates themselves, have a third digit at the beginning. If this digit is even, the route begins and ends at other interstates. If it is odd, the route has an unattached trailing end.
Interstate mile numbers start at zero at the most western or southern point, and increase towards the east or north.
Unfortunately, there is no hard and fast rule governing exit numbers, but individual states choose which of the two following options they wish to employ:
Numbered by distance is where the exit numbers correspond to the mile numbers. If there is more than one exit in a single mile, letters are affixed to the exit numbers to distinguish them.
Numbered by quantity is where the first exit is Exit 1, and the second exit, 13 miles away, is exit two. This makes more numerical sense, but new exits require either changing every exit number along the highway in the entire state, or the addition of letters, which seems okay until Exit 3 is 20 miles from Exit 3A.
The majority of interstate highways are freeways, however, some segments are tolled. These include the New York State Thruway, and Alligator Alley, I-75's jaunt from Naples to Fort Lauderdale, Florida.
If an interstate follows a diagonal route, its number can be chosen as you like it, with due regard to other Interstate numbers. For example, I-4 runs from Tampa, Florida to Daytona, Florida. Not only is it an Intrastate, but its route is primarily north - south, with east - west meanderings.
Interstate 19 is signed in metric measurements.
Interstate 238 is not a spur route of I-38, as I-38 does not exist. It's only two miles long, and resides entirely within California.
I-99 is West of I-81, and is a spur route of I-80.