Now, before you email or comment, I know there are numerous exceptions to most of what I’ve listed below! But in general, these tips will be very useful in finding your way around the Interstate Highway System. It’s the government, you know … and they don’t always follow their own rules.
- It’s a big grid.
Not a perfect grid, thanks to our geography, but a grid none-the-less. The grid consists of one- and two-digit even numbered highways running east and west, lowest numbers in the south and highest numbers in the north; and one- and two-digit odd numbered highways running north and south, lowest numbers in the west and highest numbers in the east.
- The grid is supplemented with loops, beltways, and spurs.
Spurs, likewise, are also numbered with three digit numbers, the last two of which reflect the primary Interstate from which it’s a spur, and the first digit being an odd number.
There are no duplicate three-digit Interstate numbers within one state, but the numbers are duplicated from state to state.
Loops and beltways bypass or circle major metropolitan areas, often easing your drive around highly trafficked areas. They are numbered with three-digit numbers, the first number being an even number, and the last two reflecting the primary Interstate to which they connect at two different locations.
- Mile markers are your compass.
The Interstates are indexed with small green or blue signs marking each mile of the highway — in most cases, marking each tenth of a mile. It’s a great way to know exactly where you are on the highway. Miles are numbered from the west to the east, and from the south to the north, always restarting from 0 at the border of each state. So … if you’re on I-80 (an even number so you know it’s an east-west route) and the mile markers are increasing, you know you’re going east! If you’re on I-95 and the mile markers are decreasing, you’re going south.
If you’re traveling south or west, you know exactly how many miles it is to the state border (or the end of the Interstate, if it doesn’t reach the border). Traveling north or east, however, you need to know how long that section of the Interstate is to know how far it is to the border.
- In most states, exits are numbered by the mile marker.
A few states still number Interstate exits in numerical order. But most states have followed newer federal guidelines and renumbered their exits according to the nearest mile marker, making exit numbers infinitely more useful. If you’re headed to exit 40, for instance, and you’re at mile marker 20, you know you have 20 miles to go to your exit. If the exits are numbered in consecutive order, you have no clue how far it is to the next exit based on the mile markers.
- Left exit or right exit? The clue is on the sign.
Left exits are much less common than right exits, and are often marked in a yellow section at the bottom of exit signs, also.
There are a few states who have opted not to update their exit signs to this national standard — Connecticut, for example. They have exit numbers centered at the top of the sign, giving no clue to whether it’s a right or left exit. You’ll occasionally see other exceptions, too, even in states that largely follow the federal guidelines. But for the most part, you’ll find this tip very useful.
Interstate exit signs usually mark the exits at one and two miles in advance. The exit number is at the top of the sign on a separate board. If that exit number board is flush with the left of the larger sign, it’s a left exit. If the exit number board is flush with the right side of the sign, it’s a right exit! See the photo at the right for an example of each.
- The Interstate Highway number grid is a flip-flop of the US Highway numbering system.
The US Highway system, around long before the Interstates, is also laid out in a nationwide grid. But it’s flip-flopped, north-south and east-west from the Interstate numbering scheme. Odd numbered highways still run north and south, but the lower numbers are in the east, higher numbers in the west; even numbered highways run east and west, with lower numbers in the north, higher numbers in the south.
There are exceptions to both numbering grid systems, although more exceptions in the US Highway system.
Because these numbering grids are flip-flopped, you’ll find no I-50 or I-60 route numbers; chances are they’d be in the same states with US Highways of the same route number, and would be very confusing for drivers.
- Interstate Design Standards: Roadway widths, bridge clearances and weight limits
Bridge clearances are specified to be at least 16 feet, including shoulder clearances. Although the maximum height of trucks in the US is 13′ 6″, the design standards were written to allow movement of military equipment, part of the missile defense system, now obsolete. Bridge weight limits accommodate at least maximum legal loads of 80,000 lb. (40 tons); in some states the limits are significantly higher — Michigan, for instance, with their super-heavy trucks.
HOWEVER — it’s important to note that there are many exceptions to these numbers! While new construction meets these design standards nearly 100% of the time, many older Interstates do not, especially roads such as the Pennsylvania Turnpike that were not originally constructed as Interstates but became part of the Interstate system later. Motorcoaches should not generally have a problem on any of the Interstates, but trucks do have to be careful and watch for an occasional low clearance or underweight bridge limit even on the Interstates. Exceptions should be well marked in advance of the problem. Most problems will be limited to oversize loads, not a problem for tour buses.
Interstate design standards include a minimum of four 12-foot wide travel lanes, a minimum shoulder width of 10 feet, full control of access, and design speeds of 50 to 70 miles per hour (depending on the type of terrain).
- Interstate Highway Trivia
The longest Interstate:
I-90, 3,020.54 miles, from Seattle, Washington, to Boston, Massachusetts
The shortest Interstate:
I-97, 17.62 miles, Annapolis to Baltimore, Maryland
The highest point on the Interstate system:
I-70, in the Eisenhower Tunnel at the Continental Divide in the Colorado Rocky Mountains (11,158 ft.)
The lowest point on the Interstate system:
I-95, in the Fort McHenry Tunnel under the Inner Harbor in Baltimore, Maryland (107 ft. below sea level)
Highest speed limit:
80 mph (130 km/h): I-10 and I-20 in rural western Texas and I-15 in rural central Utah
Lowest speed limit:
40 mph (64 km/h): I-490 through Rochester, New York; I-68 through Cumberland, MD; and I-394 east of I-94 in Minneapolis, Minnesota
For references and more information, visit these sites: