US Interstate Highways

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US Interstate Highways

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The Interstate Highway System is a group of limited access highways that serve the United States. Interstate highways exist in all states except Alaska and in the District of Columbia. Interstates can vary in length from under a mile to stretching across the entire country.

For many trips, the interstate highways are the most efficient way to travel by automobile between two points. Being limited access highways, interstates do not have traffic lights except on rare occasions. They typically have speed limits of 55 miles per hour (89 km/h) or more - portions of I‑10 and I-20 in rural western Texas and portions of I‑15 in rural central Utah have a speed limit of 85 mph (137 km/h) - and are well maintained.

Numbering system

The number that is assigned to an interstate highway has a specific meaning in identifying its purpose:

  • Even numbered one- and two-digit interstates under 100 transverse the country west to east. Higher numbers are further north and lower numbers are further south. Those ending in 0 are typically longer, traversing nearly the entire country.
  • Odd numbered one- and two- digit interstates under 100 transverse the country north to south. Higher numbers are further east and lower numbers are further west. Those ending in 5 are typically longer, traversing nearly the entire country.
  • Three-digit numbered interstates are generally localized highways that intersect with a one- or two-digit interstate. They share the last two digits with the main highway, and run either near or off of that highway.

This order of numbering is opposite from the U. S. Route System in order to avoid overlap of identical numbers. There is currently one instance of an interstate highway and a U.S. route of identical numbering overlapping: I-74 and US-74 in North Carolina. There is also a section of US-41 in Wisconsin that is proposed to become a section or I-41.

In Hawaii, interstates are numbered with the letter H followed by a number.

Types of signs

  • Numbering signs: Appear in the well-known interstate shield. A full sign indicates the number of the highway, the state one is in, and the direction of travel.
  • Directional signs: Indicate what exits one should take to reach a certain destination. Usually appear in green.
  • Distance signs: Indicate the distance in miles to various cities, towns, or other points along the highway or other routes off the highway. Usually appear in green.
  • Service signs: Indicate rest areas or certain service related businesses, such as food, fuel, and lodging, are coming up at a nearby exit. Usually appear in blue, and display the logos of various chain businesses.
  • Mile markers: Indicate which mile point one is at from the state line. Sometimes appear every mile, sometimes every tenth of a mile.
  • Park signs: Indicate an upcoming national or state park or historical site. Usually appear in brown.

Speed limits

Like most other roads, all interstate highways have speed limits. The limits, which are controlled and enforced by state and municipal police departments in the respective states where they are located, can vary, depending on the laws of that state, the volume of traffic, and other factors. Typical speed limits range from 55-75 miles per hour, though there are exceptions both above and below this.

In the past, it was widely accepted that there was a grace of 10 mph above the speed limit before tickets would be issued. This has changed in most states though, as excessive speed has been found to be the most common cause of fatal accidents. Now, in most states, tickets are routinely issued for speeding violations less than 10 mph above the speed limit, and in some cases, for going just 1 mph above. To avoid this, it is preferable to travel below the speed limit altogether in the presence of a police unit or out in the open where one may appear suddenly.

Other ways to avoid speeding tickets are as follows:

  • Periodically check the vehicle behind you to be sure it is not a police unit. If it is, you are safe provided you follow all traffic laws and your vehicle is not otherwise suspicious. A police unit may not always be easy to identify because different makes and models are used in each jurisdiction, and some units are unmarked. And at night, it is even more difficult to identify one due to the lack of light.
  • Watch the vehicle in front if you, as police set up speed traps ahead as well. If its brake lights turn on, it may be because of a police vehicle. For speed traps that are ahead, the police units often position themselves so they can see you before you see them.
  • Travel in the middle of a pack of vehicles. This reduces the likelihood your vehicle will be targeted, as officers can most easily spot the vehicles in the front and back.
  • If no other vehicles are on the road in your vicinity, be especially careful to obey the speed limit and other traffic laws. If there is only one vehicle in the presence of a police unit, the officer has more time to look for a violation committed by that vehicle.
  • Use the left lane for passing only, and avoid remaining there longer than you need to be. The left lane is typically the fast lane, and that is where officers look for speeders the most.
  • Use cruise control if you have it to make sure you do not accidentally exceed the speed limit. At the same time, be aware that speed increases when moving downhill, even while on cruise control, and in some places, this is where officers love to catch speeders.
  • Do not go at a speed above that of other vehicles that stands out. If you are the driver who is weaving in and out of traffic and passing all the rest, officers will take notice of this.
  • Driving a more neutral colored vehicle makes you less of a police target. Bright or unusual colors, such as red and yellow, tend to stand out more.

Radar detectors

Many motorists use radar detectors to alert them to the presence of a police unit. Contrary to popular belief, radar detectors are not always used to "break the law and get away with it." Some motorists just feel they can relieve their anxiety by knowing in advance when a police unit is around.

Radar detectors are not the ultimate machine for avoiding tickets. Police have other ways of detecting speed, and radar is not always necessary. Some police units use lasers, which cannot be picked up by radar detectors. Other places use aircraft to check speed of vehicles, and these cannot be seen on radar either. Officers also have the ability to follow a vehicle at the same speed to determine its speed.

In Virginia and Washington, D.C., radar detectors are illegal.


Tolls exist on some interstates. The tolls could be either for the highways themselves or for bridges along the highways.

Some of the tolls on Interstate Highways for cars are (this list is currently incomplete, not always up to date, and may vary from prices listed by time of day or method used to pay):

This list is incomplete. You can help WikiTravel by expanding this list.


New Jersey: George Washington Bridge has a toll of $8 while traveling in a northbound direction.

The New Jersey Turnpike from exit 7A northward is a part of I-95. The toll for this distance is $9.55.

Delaware: between exit 1 (DE-896) and exit 109 in Maryland (MD-279), there is a $4 toll in each direction. This toll is described by signs as being for the entire Delaware Turnpike, though it only needs to be paid if traveling between these two exits. The toll can be avoided southbound by taking exit 1 north, turning left on Christiana Parkway (DE-4/896), and left on Elkton Road (DE-2/MD-279), then re-entering I-95, or the reverse while traveling northbound. This takes an additional 10 minutes.

Maryland: The Millard Tydings Bridge over the Susquehanna River has a $5 toll for northbound travelers.

I-295 (Delaware)

The Delaware Memorial Bridge has a $4 toll for those traveling southbound

Rest areas

Main article: Rest area

On many interstates are rest areas where motorists can stop to take breaks. Rest areas are usually equipped with rest rooms, and many have various other facilities to meet the needs of travelers.