Knowing a bit more about the American Interstate highway system can make your next road trip go smoother.
The Interstate Highway System was formed in 1956 when President Dwight D. Eisenhower, who gets much of the credit for championing a highway system, signed the Federal Aid Highway Act. Its purpose was to create a nationwide network of high-speed motorways that had similar standards, particularly controlled access, which means no intersections or traffic signals.
Three states—Missouri, Kansas, and Pennsylvania—claim the first Interstate highway. It has since grown into a network of nearly 50,000 miles, making is the second largest in the world after China.
It’s actually a big grid.
The grid consists of more than 60 primary Interstate highways, numbered with one- or two-digit designations. Even-numbered routes run east-to-west, with the lowest numbers starting in the south; and odd-numbered routes run north-to-south, with the lowest numbers starting in the west. For example, I-10 runs between Santa Monica, California, and Jacksonville, Florida.
If you’re on a primary Interstate route that is divisible by 5, that means it’s a major route that runs cross-country, typically coast-to-coast or border-to-border. For example, I-95 runs between Miami, Florida, and Houlton, Maine.
The grid is supplemented by a number of loops and spurs called auxiliary highways that are designated by three-digit numbers. For example, I-110 is a spur off I-10 near Los Angeles. Auxiliary routes that begin with an even number generally connect to the main highway at two places, while routes beginning with an odd number connect at only one location. Auxiliary highways that encircle a metropolitan city, such as I-495 that loops all the way around Washington, D.C., are known as beltways.
Mile markers are your guide.
Mile markers along the Interstate can help you tell which way you’re going and where you are along the route. They always start at 0 at the state border and are numbered from west to east or from south to north. So if you’re traveling on I-75 (which you know is a north-south route because it is odd-numbered), and the mile markers are increasing, you know you’re traveling north. The miles on the markers tell you how far the southern border is behind you, and if you know about how many miles long the state is, you can calculate the number of miles until you reach the northern border.
Exit numbers reveal distance.
Most states have adopted modern federal guidelines, so the exit numbers correspond to mile markers. There are a few states where the exit numbers are simply sequential. In places where the exits are numbered by the mile marker, you can use exits to determine your distance. If you know you need to get off the Interstate at Exit 87 and you just passed Exit 52, that means you have 35 miles to go.
Exit number signs, which typically appear on top of highway signs, provide clues about which side of the Interstate the exit ramp is on. If the exit number is flush right with the sign, the exit ramp is to the right. If the exit number is flush left, it’s a left-side exit.
States set the speed limits.
Speed limits are determined by individual states, and they are typically based on whether the roadway is in an urban or rural area. Speed limits generally range from 50 to 75 mph. The highest Interstate speed limit is 80 mph, which can be found on specified segments in Idaho, Montana, Nevada, South Dakota, Utah, and Texas. A state highway in Texas has a posted speed limit of 85 mph, the highest in the country.
Many GPS systems now display the speed limit of the road you’re traveling, and the Waze driving app will alert you if you’re speeding.
Speed limits are absolute in most states, which means if you’re going 41 mph and the sign says 40, you have violated the law. However, general wisdom suggests that highway patrol officers won’t pull you over for going up to 9 mph over the speed limit. There’s no guarantee for this, and road conditions could be a factor, so follow this legend at your own risk.
Wait, Hawaii has Interstates?
Yes, there are Interstate highways in Hawaii, Alaska, and Puerto Rico despite that they don’t connect to other states or territories. The designation indicates that the roads receive federal funding. But only Hawaii’s have federal Interstate highway signs—its routes have an H prefix. You can find H1, H2, and H3 on the island of Oahu.