Interstate Signs—Did You Know?
We are almost overwhelmed by road signs. Our well-meaning federal and state governments—like any bureaucracy—has posted a gazillion signs all with the underlying purpose of helping us find our way, find our location, find where we have been, find where we are going, or find where we are right now. Whew! Even with the help of a GPS, we use road signs. Let’s take a look at some road signs, get some help, maybe learn something, and have some fun, too. Remember the old adage… “Latitude is Where We are Lost. Longitude is How Long We’ve been Lost There!”
Most of us drive on the Interstate highways scattered all over the USA and are used to all those “green” signs from the small, ubiquitous, “Mile Marker” to the giant Route signs. We use them, trust them most of the time, and depend on them to guide us to where we want to go. Here is some little-known information about those signs that may help you in the future…
Mile Markers… mark miles, of course. Their numbers start at the state line when you cross into a new state or they may also start at the beginning of that specific Interstate. For example, I-30 starts just west of Fort Worth (MM 1) and connects Dallas/Fort Worth with Texarkana, Texas and North Little Rock, Arkansas. I-30 ends when it intersects with I-40 in North Little Rock. The last mile marker on I-30 is MM 142 or 142 miles east of the Texas/Arkansas state line.
For east/west highways, mile markers count from west-to-east. If you are driving eastbound, they start with “MM 1” one mile from the state line (“MM 0” is rare). They start counting from south-to-north for highways going in those directions. When driving north, numbers start one mile from the southern state line.
Usually, Exit numbers correspond to the mileage markers on the Interstates. If you are in the middle of nowhere and need assistance, providing emergency personnel with the nearest mile marker can be crucial to finding your location. This is why you should pay attention to the mile markers during your trip. Some states have mile markers every 2/10 of a mile or five markers per mile. Some of those states have learned to put these multiple-markers in the median with the mileage information on both sides. This saves doubling the number of signs when they are positioned near the right shoulder.
Just for Fun… What is the highest numbered mile marker in the USA?
Exit Numbering… in most states is based on the mile marker. This system is extremely helpful to the driver. For example, if you need to get off at Exit 80 and you are at MM 70, you know you have ten miles to go. A few states continue to number their exits sequentially and do not follow the federal guidelines. This is extremely confusing to anyone visiting the area.
The Highway Numbering System… In the system of numbering Interstate highways, simply keep in mind… odd and even numbers. First, highways that are east/west are assigned even numbers and north/south highways are assigned odd numbers. One way to remember this is to think of “E” for “Even” and “East.”
Also, the Route Numbers of even-numbered routes increase from south to north, For example, in the southern USA, I-10 (lower number) runs across the nation between Santa Monica, California and Jacksonville, Florida. In the northern USA, I-90 (higher number) runs across the nation between Seattle, Washington and Boston, Massachusetts.
Odd route numbers increase from west to east. For example, major north/south Interstates running between Canada and Mexico increase their route number from west to east. There is I-5 running along the West Coast and I-95 along the east coast.
Any route numbers that are divisible by 5 (such as I-10, I-90, I-5, and I-95 mentioned above) are intended to be major arteries among the primary routes. These routes are designed to carry traffic long distances.
[Author Note… There are exceptions to all of this but they are somewhat rare.]
The federal Interstate highway numbering system is a grid overlaying the nation that uses a system similar to what the US highways used prior to the Interstates, except it is flip-flopped. What about I-50 or I-60? There are no Interstates with those two numbers. When they flip-flopped the system, an Interstate highway with those numbers would go through states that already had a US-50 or US-60. It would be confusing. We do have an I-10, I-20, I-30, I-40, I-70, I-80, and I-90 now. Hey, it works!
There is a group of shorter highways called “Auxiliary Interstate Highways” and they are considered circumferential, radial or bypass, and spur highways. This means (A) they principally serve urban areas, (B) they encircle a city (I-465 goes around Indianapolis), (C) they partially encircle a city (I-895 in Maryland is a bypass of downtown Baltimore), or (D) they are an offshoot that goes some specific place (I-110 in California links I-10 with the Port of Los Angeles). The Auxiliary Interstates have a 3-digit number. The first digit is usually tagged onto a local, major Interstate route number.
One of the best and most useful, but little known, things about signage on Interstate highways is that the positioning of the Exit Number sign over the Route sign is an indication of whether the exit location is to the right, left, or straight. About one or two miles before an exit, a large Exit sign will be mounted up overhead (such as the ones shown below), in the air, to indicate what town and/or route number (if appropriate) will be at the upcoming exit.
Notice the picture above…
- Exit 2G sign is mounted on the left side of the Route sign—this means it is a left exit.
- Exit 2E sign is mounted on the right side of the Route sign—this means it is a right exit.
- Exit 2H sign is mounted and centered on the Route sign—this means stay in that lane.
- The remaining Route sign has no exit sign—this means continue on this highway.
You will find that some states have not adopted this guideline and the result is always confusing for a driver unfamiliar to the area. While left exits are uncommon, some do exist in about every state. Driving a big rig on a crowded highway, you always appreciate knowing what lane position you should be in ahead of time.
Just for Fun… What does WW or B or AD mean to you?
Unusual Route “Numbers”
Although not truly Interstate-related, recently, we drove through Missouri and noticed some unusual (to us) route designations for local highways. Missouri (I’m not sure about other states) uses letters instead of numbers to designate numerous local routes. While this, I’m sure, is perfectly clear to the residents, it is confusing to those of us just passing through. Shown below are two of the route markers we passed while driving through the state…
Although we did not have to exit onto any of routes marked like this, I was watching the map and the signs and was just curious about them. One unusual thing I learned is that the various letters used for designating these local routes are duplicated in various places all over Missouri. That sounds like the basis for another article.
Hope We Helped
Knowing a bit more about some of the little known background and intentions of the Interstate system of highways may make it a bit easier for you to understand why certain things are that way. While RVers often avoid the Interstate highways in favor of slowing down and taking in the local sights, our highway system is excellent and will get you there efficiently.
Oh, and by the way, the highest numbered mile marker in the USA is on I-10, going east, at the last exit in Texas before you enter Louisiana. You have to admit, 880 miles (1,416 km) is a long drive. There is a Flying J truck stop on this highway at Anthony, Texas, at MM 0 at the New Mexico/Texas border, about 18 miles west of El Paso.