United States

On Interstate highways in the United States, speed limits range from urban limits as low as 40 mph (65 km/h) to rural limits as high as 75 mph (120 km/h). Before the 1973 energy crisis, some states posted no speed limit on Interstate highways. At one time Kansas had an 80 mph (130 km/h) speed limit on its turnpike system. In 1974 Congress imposed a nationwide 55 mph (90 km/h) speed limit by threatening to withhold highway funds from states that did not adopt this limit. It was estimated a speed of 55 mph used 17% less fuel per mile than a speed of 75 mph (120 km/h). It was also believed, based on a noticeable drop the first year the limit was imposed, that it cut down on highway deaths, but later studies were more mixed on this point. This limit was unpopular, especially in Western states. In 1987 states were permitted to raise speed limits to 65 mph (105 km/h) on rural Interstate highways.

All federal speed limit controls were lifted on November 28, 1995, leaving the task of setting maximum speeds to the states. Immediately, Montana reverted to non-numerical speed limits on most rural highways. Many other states reverted to older policies allowing speed limits higher than 65 mph (105 km/h). However, no Interstate highway, freeway, or expressway is currently signed for over 75 mph (120 km/h), and within major city limits, few freeways have speed limits over 65 mph (105 km/h).

California

In California many speed limit signs are identified as "Maximum Speed", usually when the limit is 55 mph (90 km/h) or more.

Kansas

The Kansas Turnpike used to have an 80 mph speed limit. No Kansas road currently has a speed limit higher than 70 mph.

Montana

It is a myth that Montana had no speed limit. In fact, for four years, it had a "reasonable and prudent" speed limit during the daytime on most rural roads. At night, these roads had a night speed limit, usually 65 MPH or 55 MPH, depending on road type. In some cases, the police enforced an approximately 90 mph (140 km/h) speed limit as "reasonable and prudent."

In a challenge to a speeding ticket, the "reasonable and prudent" laws was found to be too vague to be constitutional. Thus, for most of the first half of 1999, there was no speed limit whatsoever on most rural Montana roads.

In June 1999, a new Montana speed limit law went into effect. The law's practical effect was to require posted limits on all roads and disallow any speed limit higher than 75 mph (120 km/h).


Texas

Typical Texas rural speed limit sign. Note the black back grounded 65 mph night speed limit sign, common on Texas roads. (Few other states have widespread night speed limits.) This sign is on southbound U.S. 69/96/287 just north of Beaumont.

Texas is the only state whose speed limit laws and rules generally do not prescribe a specific limit for each type of roadway. Any rural road—two lane, four lane, Interstate, or otherwise—that is numbered by the state or federal government has a 70 mph (110 km/h) statutory limit. The law generally allows changing the 70 mph limit only if a study recommends a different limit.


75 mph and 80 mph limits

In 2001, the Texas Legislature allowed the Texas Department of Transportation to post 75 mph (120 km/h) speed limits in counties with fewer than 10 people per square mile. This has the practical effect of only allowing 75 mph speed limits in the most sparsely populated counties, all of which are generally well west of a line stretching from San Antonio to Odessa. In 2005, the Texas Legislature revised this law, allowing 80 mph (130 km/h) limits on I-10 and I-20 in certain rural counties in west Texas. This law also revised the eligibility for 75 mph speed limits: now eligible counties can have up to 15 persons per square mile.

Texas law does not disallow 75 mph speed limits on two-lane roads. Several west Texas 2 lane roads carry 75 mph limits, including portions of US 90. No other state has a limit higher than 70 mph on any 2 lane road.


Environmental speed limits

Texas is the first state to lower speed limits for purportedly environmental reasons since the 1995 repeal of federal speed limit controls. In roughly a 50-mile (80 km) radius of the Houston-Galveston and Dallas-Ft. Worth regions, the Texas Commission on Environmental Quality co-opted the Texas Department of Transportation to reduce the speed limit on all roads with 70 mph (110 km/h) or 65 mph (100 km/h) speed limits by 5 mph. This was instituted as part of a plan to reduce smog-forming emissions in areas out of compliance with the federal Clean Air Act.

Initial studies suggested the lower speed limits could bring the areas roughly 1.5% closer to compliance. However, follow-up studies suggest that the actual benefit is only a fraction of this original estimate. First, the emissions modeling software used for intitial estimations, MOBILE 5a, overestimated the emissions contribution of speed limit reductions. Rerunning the models with the next generation software, MOBILE 6, produced dramatically lower emissions reductions. Second, speed checks in the Dallas area performed 1 year after implementation of speed limit reductions show that actual speed reductions are only about 1.6 MPH, a fraction of the anticipated 5.5 MPH speed reduction. With both of these facts combined, it is possible that the speed limit reductions only provide a thousandth of the total emissions reductions necessary for Clean Air Act compliance.

The Houston-Galveston area briefly had all roadways capped at 55 mph (90 km/h) in mid-2002. Facing immense opposition, poor compliance, and the realization that lowered speed limits produced only a fraction of the originally estimated emissions reductions, the TCEQ relented and reverted to the 5 mph reduction scheme.

In 2003, the Texas Legislature prospectively banned environmental speed limits effective September 1, 2003. The wording of the bill allows environmental speed limits already in place to remain indefinitely; no new miles of roadway may be subjected to environmental speed limits, however.

This law has allowed interesting inconsistencies. Generally, all primary arterial roadways within the inner loops of Texas cities have speed limits of 60 mph (95 km/h) or lower, so they were not subjected to environmental speed limits. Arterial roads between the inner loop and the outer loop generally have 65 mph (100 km/h) limits, and arterial roads outside the outer loop generally have 70 mph (110 km/h) limits. (Note that this "standard" is only an observed pattern. It is not prescribed by law.) In at least one case—TX 121 between I-35W and I-820 in Ft. Worth—the speed limit rises from 60 mph to 65 mph as one crosses IH-820 approaching downtown, contravening the standard.


Night speed limits

Texas is the only state with a broadly applicable night speed limit. Texas statutorily prescribes a blanket 65 mph (100 km/h) night speed limit on roads with a speed limit of at least 70 mph (110 km/h). While the Texas Department of Transportation has the power to raise or lower this night speed limit, it in fact rarely does, so nearly every 70 mph or higher speed limit sign has an accompanying 65 mph night speed limit sign.

North Dakota, Montana, and Oklahoma also have night speed limits, but they are only generally applicable to rural, non-Interstate-class roads.


Minimum speeds

In addition to the legally defined maximum speed, minimum speeds may be posted. There is little evidence to suggest they are enforced. In California the minimum speed by regulation on freeways (in free flowing traffic) is 45 mph, although this is generally not posted.

In general

On US roads, the speed limits are usually as follows:

* 15–25 mph (25–40 km/h) in school zones
* 25–30 mph (40–50 km/h) on residential streets in cities and towns
* 35–45 mph (55–70 km/h) on major arterial roads in urban and suburban areas
* 45–70 mph (70–110 km/h) on highways outside cities and towns and urban expressways
* 55–70 mph (90–110 km/h) on non-Interstate freeways and rural expressways.
* 65–75 mph (105–120 km/h) on rural Interstate freeways

Generally, western states have higher limits than eastern states.

For a current listing of all U.S. State Highway Speed Limits click on Speed Limits by State, Insurance Institute for Highway Safety