Driving Interstate Highways

Tips to help people driving
Interstate Highways in the United States of America

*Interstate Highways*

This page presents information that can help people driving Interstate Highways in the United States of America.

Disclaimer: I neither work for or with any transportation department. My readings and observations from driving over many years follow. I guarantee nothing.


*Interstate Highway Numbers

For the federal interstate highways (highways with the red, white, and blue shields), even numbered highways run east/west and odd numbered highways run north/south. Interstates 25 and 40 in Albuquerque NM are classic examples of this numbering system.

The direction of an interstate is determined not by its local direction, but by the starting and ending points of the entire interstate. It is possible, even likely, that a north/south interstate may go east or west for awhile to go around a mountains, rivers, cities, or other large objects. You ignore such wiggles when stating the direction of the interstate.... keep true to its overall direction.

The exceptions to these numbers are highways with three-digit numbers (101 to 999). These are "bypass" highways that connect to, or are associated with, a main interstate (e.g., 405 is a bypass for interstate 5). Bypasses can be perpendicular or parallel to their associated interstate. Bypasses can even meet their counterpart head on (in New Jersey, 295 SOUTH meets 95 NORTH, while traveling east/west at the time... causes endless confusion to the unwary). The same bypass number can be used in different states for different bypasses An "I405" bypass is found in both California and the state of Washington.

If the hundreds digit of a bypass (the "4" in "405") is even, then it is likely (at least planned) that after the bypass splits off from its parent highway, the bypass will join up with its parent again. Somewhere. If you are going in the right direction and construction is finished. If the hundreds digit is odd, then the bypass was not expected to reconnect with the parent highway.

If you must guess at the direction of a bypass from its number first look if the number is even or odd then look at the 100's digit. If the 100's digit is even, guess at the same direction as the main highway. Else when the 100's digit is odd, guess a perpendicular direction, or something close to perpendicular. Examples: 405 may be north/south, just like the 5. 305 may go east/west, angled to the 5.

For one and two digit interstates whose numbers are odd, the lower the number the further west the highway is. For even numbered interstates, the lower the number the further south the highway is (this follows the same direction rules exit numbers and mileposts do). Further, interstate numbers ending in 5 or 0 tend to cross the entire available span of the United States, or nearly so.

As far as this author knows, state or local highway numbers do not follow the convention for interstate numbers.


Interstate Exit Numbers

For states blessed with exit numbers, there are two styles of exit numbers. Typically a state chooses one style for use everywhere. Some states, such as New Jersey, use both on different highways.

Simple Exit Numbers

In this system the first exit is exit 1, the second is exit 2, etc. Exit numbers usually increase as you head north or east. Watching how exit numbers change as you drive by them tells you what direction you are traveling.

New exits added between existing exits have letters after the exit number. "Exit 8A" would be between exits 8 and 9.

Mile Number Exits

Each exit is assigned a number that shows how far it is from other exits. Exit numbers usually increase as you head north or east. Watching how exit numbers change as you drive by them tells you what direction you are traveling. If your are at exit 203 on I25 and are after exit 230, you know you have about 27 miles of northbound driving left.

Also see the following section on Mileposts.

While the Pennsylvania turnpike started off with simple exit numbers, newer exits seem to be using milepost exit numbers.



Most states place "mile posts" (also called "mile markers") along the side of their major highways and interstates. While there are no national requirements on mileposts many states use green paddles with white numbers on them. Highways begin at milepost "zero". For Interstate highways this is at its west-most or south-most starting point. Other highways, such as New Jersey's Atlantic City Expressway, may use other directions (regardless of looks, such are not interstate highways).

From mile zero mileposts increase each mile driven. Along with identifying your current position, watching posts change as you drive by them also provides your overall compass bearing (north/south or east/west). Mileposts are usually placed accurately enough to check car's odometer with on straight roads.

When reporting road problems to the local authorities, providing the exact position using mileposts is usually appreciated ("traffic accident between mileposts 195 and 196", or better yet, "at milepost 195.7"... while there may not be at ".7" post, adding the tens digit places the problem very accurately.

When interstate exits are marked with the exit numbers based on mileposts (see previous Mile Number Exits), the exits numbers for both directions are ALWAYS the same. The exit number is determined by where the south-most or west-most exit ramp starts.

California has one of the weirdest milepost systems I've seen. It appears to me that these posts are not intended to help drivers navigate the highways but help road maintenance and police reports. California posts are white paddles with small black letters. Their lettering is small and hard to see while driving. Drivers trying to watch the posts and the road at the same time will find these signs dangerously small. At the top of the paddle is a two or three letter code showing the county the post is in (SD for San Diego, RIV for Riverside). Below this is the mile number, relative to the county. The numbers reset to zero in each county. Further, posts are placed wherever they fall and you get numbers like "23.57". Bridges and overpasses are often marked with a sign that also has the mile, as well as other information, on it.


Toll Roads

A few words to describe the various tolls one encounters when driving the United States of America. There are three types of tolls:
  1. Single fee tolls: used on bridges, tunnels, and other short runs that have high expenses. Sometimes the toll is only charged for one direction of travel, relying on the return trip of the typical driver to average costs out.
  2. Ticket method: cash paying customers grab a ticket when they get on the highway and hand the ticket in when they get off.

    The face of the ticket typically has all of the exit numbers, appropriate names, and required fee, for each exit. The back of the ticket may contain other information, including fees for trailers, trucks, or other special vehicles.

    Keep that ticket in a very safe place. If you loose the ticket, you pay the maximum possible fee for that exit. Young children should not hold the ticket for you!

    If you need to fumble around putting change away after exiting a ticket based tollbooth, pull away a little beyond what is needed to let the next driver start their payment, stopping before you enter general traffic, and then do your fumbling. This should only be short-term delay as the person behind you may well have exact change and be through quickly. Also, do not try this on anything but ticket tolls were all prices are different; other toll systems are much quicker to pay making it very likely you will be blocking the car behind you.

  3. Toll Plaza method: a less popular method with drivers is to have a toll plaza every so many miles where drivers must slow down and throw some money into the system before continuing on. Regular interruptions to throw money into "piggy banks" is not popular with long distance drivers nor is paying a full fee if you get on at one exit, pay a toll on the road, then get off at the next exit.

    Many piggy banks are automated, and that presents several problems. Most toll machines do not show you how much money you have put in or return any change. For change, or receipts, you must use manned lanes.

    Entrances and exits at spots in between the main toll plazas can have smaller tolls to account for the shorter distances between them and the main toll plaza. The difficulty is these may not be manned all the time, yet you still must pay a fee. This can present problems to people without exact change. If you are stuck trying to get off at such exits without exact change see if there is an envelope you can take to mail in the toll at a later time. If such envelopes exist, read the instructions by them on how to get any tollgate open. Another technique is for the toll machine to spit out a receipt that has all information required for payment. No matter how it is done you can be sure the road people will want their money. Look around, you will find something.

    Tollbooths typically have some type of light that tells drivers when the toll has been accepted. Wait for this light before continuing on.

    If you have problems with automated coin systems follow the "official" procedure, which may well be posted on signs by the exit. Honking your horn then waiting a specified time before continuing is a typical response.

Distances between exits on toll roads, or on Interstates in less populated areas, can be surprisingly long. Drivers need to be aware of signs that state things like "next exit 30 miles", "next rest area 76 miles", and blue food and gas signs.

Some electronic toll systems, such as EZ-Pass, exist allowing subscribers to charge their credit cards rather than paying cash. Unless you are a repeat user this author suggests you avoid these things. Some toll lanes may only accept this electronic form of payment. Unaware drivers expecting to pay cash can have great difficulty if they find themselves in this lane.

Some toll highways may also have their own tokens that may be used instead of money. Again, there may be "token only" lanes that get cash customers into trouble. Tokens tend to limit themselves to fixed-fee tolls, such as the Single Fee Tolls, and Toll plaza methods of collecting tolls.

When approaching a toll plaza you can not assume all the lanes are the same. As some lanes may well not accept the cash in your hand for payment, you must be sure to be in the appropriate lane for your payment method. This can be made extra difficult if the approach to the toll plaza is a curved road where you can not easily match the big signs over the tollbooths with the lanes you are in. If you are lucky lane numbers will be painted in the road.

There is no good place to fumble for change or tickets as you approach a toll plaza. If I need to fumble, I tend to pick the lanes with the longer lines just go give me more fumbling time.

Tollbooths for long distance highways may have pull off areas before or after the plaza. Sometimes both before and after. These areas may provide pay phones at window level as you are not to get out of your vehicle in these areas. Without restrooms and rest areas these are not for resting, but only short delays. Then again, the toll plaza may not provide these services. If you want to use such an area, keep your eyes open for them and ask a toll collector about where you can find them.


Interstate Myths

One Mile In Five is FALSE:
The Eisenhower Interstate Highway System requires that one mile in every five must be straight. These straight sections are usable as airstrips in times of war or other emergencies.
Richard F. Weingroff debunks this myth at http://www.tfhrc.gov/pubrds/mayjun00/onemileinfive.htm and http://www.washingtonpost.com/wp-dyn/articles/A21527-2005Feb13.html.


. . . . . .. . . . . .Written by Gilbert Healton ().


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