Updated: Aug 18, 2019
Have you ever dreamed about leaving everything behind for an extended, care-free, cross-country road trip where you have no specific destination in mind? Or do you hate relying on your GPS for long periods of time or long-distance driving because of data usage? With basic knowledge of the interstate highway system, you can do most of your interstate driving without the aid of a GPS!
The interstate highway system began construction in 1956 at the request of President Dwight D. Eisenhower. He recognized the benefit of such a highway network after serving in World War II and experiencing the Reichsautobahn system in Germany firsthand and noting its usefulness in rapidly transporting supplies long distances. Nowadays, they provide a great (and safe) way to transport ourselves long distances on epic roadtrips, as they require essentially zero traffic stops (which came in handy the time that I accidentally drove from Indiana to Alabama with depleted front brake pads) and prohibit pedestrian and non-motorized vehicles (reducing the number of accidents).
Additionally, the well-designed numbering system of both the highways and the exits allow a driver to easily know their direction of travel and location along the route and quickly estimate arrival times to their destination. Understanding how the numbering system works will allow you to travel anywhere without heavily relying on a GPS or map.
All primary interstates in the continental United States are assigned a one- or two-digit number. Interstates that primarily run east-west are assigned even numbers and interstates that run north-south are assigned odd numbers. Odd-numbered interstates increase from west to east, while even-numbered interstates increase from south to north, as shown by the map below.
So if you are wanting to travel from Seattle (intersection of I-5 and I-90) to Houston (intersection of I-45 and I-10), you generally would want to take odd-numbered interstates south until you reach I-10 and take even-numbered interstates east until you reach I-45. As you can see, outside of the northeast U.S., the interstate system is easy to navigate, with only a handful of east-west and north-south routes available. The interstate system in the Northeast U.S. is a little more complex, but becomes easier with experience and paying attention to signage.
When you drive through a large city, you’ll likely encounter interstates with three digits. Three-digit numbers are reserved for spurs, loops, connections, and bypasses. If the first digit is even, the route is a connection, loop, or bypass which reconnects to a primary interstate (typically the one that shares its last two digits) as its endpoint, whereas an odd first digit indicates a spur which does not reconnect to a primary interstate as its endpoint (although it may intersect a primary interstate). The example below shows Los Angeles and the surrounding area, including San Bernardino. I-10 and I-15 intersect in San Bernardino, with a bypass designated as I-215. Only two primary interstates, I-5 and I-10, intersect in Los Angeles, but six three-digit routes are also present, with I-210, I-405, I-605 as bypasses and connections and I-105, I-110, and I-710 as spurs. Note that I-210 becomes state highway 210 just west of San Bernardino, but connects the easternmost endpoint of I-210 shown here to I-10 just east of its I-215 interchange. While this roadmap looks fairly complicated, it provides an easy way to navigate through busy cities. For example, if you wanted to drive from San Francisco to San Diego, you could take the I-405 bypass to avoid bad traffic along I-5 in Los Angeles. Or, if you needed to head east towards Phoenix, you could take the I-210 bypass to again avoid the downtown area.
Although you can now navigate the national interstate system, you’ll also need to determine when you need to exit the interstate to merge onto a different interstate to change your direction of travel or just stop for gas or food. This is easily done by taking note of exit numbers. Similar to primary interstates, exits are numbered in every state starting at Mile 0 at the southernmost (for north-south routes) or westernmost (for east-west routes) point along that route and increasing towards the east or north. Thus, if you are traveling west or south, you know exactly how many miles until you enter a new state. Additionally, if you are looking for an exit numbered 30 and you just passed exit 100 traveling 70 mph, you can estimate that you will reach your exit in approximately 1 hour. If you realize that you only have enough gas to drive another 50 miles, you’ll need to stop at an earlier exit to fill up.
Two final things to note: Exit numbers on overhead signs, often at interchanges where multiple interstates meet, indicate which lane you need to be in by the position of the exit number with respect to the rest of the sign. An exit number on the left side indicates that you need to be in the left lane, whereas an exit number on the right indicates that you need to be in the right lane. This is quite useful, since many different interchange designs exist, and the exit direction does not always correlate to the planned direction of travel.
Finally, a little review of driving etiquette. Interstate lanes are designated for passing, traveling, and entering/exiting the highway. Typically, the far left lane is reserved for passing, meaning that you should not be in that lane if you are not intending to pass anyone. Doing so increases the risk of road rage and traffic accidents (studies show that driving 5 mph under the average speed of traffic in the left lane is far more dangerous than driving 5 mph over the average speed of traffic in the left lane) and in most states is illegal. Therefore, always leave the left lane for passing and the right lane for entering/exiting the highway, and travel in the center lanes. When only two lanes are available, the traveling lane is also the right lane.
With these tips in mind, take to the roads with confidence on your next road trip. If you need road trip ideas, check out the Ultimate Road Trip Series!