Friday, November 19, 2010

Interstate Intelligence

Dear Joe and Vicki: A fellow RVer tried to explain the interstate numbering system to me but I'm still confused. Just what is the system and why is it important to me?

Joe: Knowing how to interpret the interstate numbering system, its mile markers, and exit numbers can be a valuable tool for the RV traveler. The numbers on the interstate signs provide information about your location, direction of travel and distance to your destination. Interstate numbers, mile markers and exit numbers are frequently used on road signs, billboards and radio station advisories. And knowing the numbering system will help you furnish directions when you call for roadside or emergency assistance.

Interstate highways are described as traveling either east-and-west, or north-and-south. There may be areas where an interstate does not run exactly due east/west or north/south but the main direction that most of the road travels is the one used to describe it.

The east/west interstate routes are even-numbered with one or two digits. The numbering system of the east/west interstates begins with the lowest (I-4 in Florida) and progresses to the highest interstate number (I-96 in Michigan).

The north/south interstate routes are odd-numbered with one or two digits. The numbering system of the north/south interstates begins with the lowest (I-5) on the west coast and progresses to the highest interstate number (I-95) on the east coast.

There are also shorter, three-digit interstates. Three-digit interstates usually connect other interstates or provide a loop around a city.

Now you know - if you are on an even-numbered interstate you are traveling either east or west; if you are on an odd-numbered interstate you are traveling either north or south. And if you are on a three-digit interstate, you might be driving in a circle.

Vicki: Interstate highways have mile markers; those little green signs located on the right side of the highway. The signs have a number and possibly the word “mile” written on them.

Mile markers on east/west interstates begin counting from the state’s western state line. Mile markers on north/south interstates begin counting from the state’s southern state line. When an interstate highway originates within a state (I-20 in Texas for example) the numbering begins at the southern or western location where it begins (in this case, at I-20's junction with I-10). So mile marker numbers get higher as you travel north or east. When you cross the state line into another state, the mileage numbers start over again.

The three-digit interstates that form a complete loop (circle) around a city have mile markers that are numbered in a clockwise direction. The numbers begin just west of the spot where an interstate meets the southernmost point of the loop. I-465, for example, is a 53-mile loop around Indianapolis. Mile marker 1 is just west of where I-65 intersects the southernmost point of I-465’s loop. Mile marker 53 is just east of this same intersection.

Most states number their interchanges and exits so they match the nearest mile-marker number. Exit 12, for example, will be very close to mile-marker 12. If you are looking for Exit 12 and you have just passed mile-marker 10 you know your exit is only two miles away.

There are still a few states, however, that number their interchanges and exits consecutively without linking them to the mile markers. In other words Exit 12 may be the 12th exit from the border; but not necessarily at the 12-mile mark. Look at a map to determine which system is being used to mark the interchanges and exits. Better yet, pay attention to the mile markers and exit numbers to see if they match.

Using the interstate numbering system simply amounts to knowing:

Even-numbered interstates go east and west. Odd-numbered interstates go north and south.

When you enter a state from the south or west, the mile markers (and usually the exits) will begin with 1 and get higher as you travel.

When you enter a state from the north or east, the mile-marker numbers (and usually the exit numbers) will be high and get smaller as you travel.

Next time you are driving on an interstate highway pay attention to the highway number, mile markers and exit numbers. Then ask yourself: what highway are you on, what direction are you headed, and what is the nearest mile marker? Those are the questions you will be asked when you report an emergency.

Joe and Vicki Kieva are the authors of a number of how-to books and e-books about RVs, RVers and RVing.

Return to RV Know How