Speed limits in the United States

Main article: Speed limit

A standard sign indicating a speed limit of 80 miles per hour (mph), a night-time speed limit of 65 mph, and a truck speed limit of 55 mph

Speed limits in the United States are set by each state. Speed limits are usually:

• 25–30 mph (40–50 km/h) on residential streets
• 35–45 mph (55–70 km/h) on urban arterial roads
• 50–65 mph (80–105 km/h) on major highways inside cities
• 45–65 mph (70–105 km/h) on rural two-lane roads
• 55–70 mph (90–110 km/h) on rural expressways
• 65–80 mph (105–130 km/h) on rural Interstate highways

Western states generally have higher limits than eastern states, and some limits fall outside these ranges. For example, some two-lane rural roads in Texas have 75 mph (120 km/h) speed limits.

Chart

This table contains the usual speed limit, in miles per hour, on typical roads in each category. This is usually, but not always, the statutory speed limit.

Some states have lower truck speed limits applicable to heavy trucks.

State	Interstate (rural)	Interstate (urban)	4 lane (rural)	2 lane (rural)	County (rural)	Residential (urban)
Alabama	70, 55hazmat	55	65	55	45paved, 35gravel
Alaska	65	65	65	65	50
Arizona	75	65	65	65	55
Arkansas	70	65	65	55
California	70	65	65	55	65	25
Colorado	75, 65mtn	55	65	65
Connecticut	65	55	65	55
Delaware	65	55	55	55
District of Columbia	N/A	55	N/A	N/A	N/A	25
Florida	70	65	65	60	55
Georgia	70	55	65	55	45	30
Hawaii	60	50	45	45	45
Idaho	75	65	65	65
Illinois	65	55	65	55	55
Indiana	70	55	60	55	55
Iowa	70	55	65	55
Kansas	70	65	65	65
Kentucky	65	55	55	55	55
Louisiana	70	60	65	55
Maine	65	55	55	55	50
Maryland	65	55	55	55
Massachusetts	65	55	55	55		30
Michigan	70	65	55	55	55	25
Minnesota	70	65	65	60	55	30
Mississippi	70	70	65	55
Missouri	70	60	65	60	55 (lettered roads)
Montana	75	65	70	70
Nebraska	75	60, 65lin	65	65west, 60	55paved, 50gravel	25
Nevada	75	65	70	70	70	30
New Hampshire	65	55	55	45	35	30
New Jersey	65	55, 65	55	50		25
New Mexico	75	65	70	65
New York	65	55, 50nyc	55	55	55	30
North Carolina	70	65	70	55
North Dakota	75		70	65	55	25
Ohio	65	60 or 65, 55truck	55		25
Oklahoma	70, 75t	60	65	65	45	25
Oregon	65	55, 60	55	55	55
Pennsylvania	65	55	55	55
Rhode Island	65	55	55	55
South Carolina	70	60	60	55	45	30
South Dakota	75	65	65–70	65	55	25–35
Tennessee	70	55	65	55
Texas	70, 75, 80	60	70, 75	70, 75	60, 55night	30
	All roads have 65 mph night limit if day limit is higher. 75 mph only in counties with fewer than 15 people per square mile. 80 mph only in west Texas counties specified by statute. Truck limit is 70 if regular limit is higher. Harris County may post up to 70 mph limits on the Harris County Toll Road Authority tollway system.
Utah	75	65	65	65
Vermont	65	55	55	50	50
Virginia	70 (I-85), 65	60, 55	60 (US 29, US 301, US 58), 55	65	55	25
Washington	70	60	60	60	50	25
West Virginia	70	55	65	55
Wisconsin	65	55	65	55	55	25
Wyoming	75	60	65	65

Legend

• Interstate: Interstate highway or other state- or federally-numbered road built to Interstate standards.
• 4 lane: State- or federally-numbered 4 lane road not built to Interstate standards.
• 2 lane: State- or federally-numbered 2 lane road.
• County: County-owestd roads that are generally not numbered by the state.
• Residential: Residential roads within an urban area.

Footnotes

• gravel: Gravel roads.
• hazmat: Trucks hauling hazardous materials. Only applies to four-lane and Interstate roads in Alabama.
• lin: Interstate 80 through Lincoln.
• mtn: Speed limit in mountainous areas.
• night: Night speed limit.
• nyc: New York City freeway speed limit.
• paved: Paved roads.
• t: Turnpike.
• west: West side of state.

Definition of speeding

Speeding is defined by the US Federal Government as either exceeding posted limits or driving too fast for conditions. Speeds in excess of posted maximum speed limits account for most speed-related traffic citations. Most speed-related crashes involve speed too fast for conditions such as limited visibility or reduced road traction. Variable speed limits offer some potential to reduce speed-related crashes, but due to the high cost of implementation exist primarily on motorways, while most speed-related crashes occur on local and collector roads Speed-related crashes can also occur at speeds below 30 miles per hour; for example, truck rollovers on exit ramps.

Prima facie

In certain instances, some United States speed limits are prima facie. In practical terms, this means that a legitimate defense to a charge of speeding is that one's speed is actually reasonable and prudent despite the limit. The definition of reasonable and prudent is highly subjective, however, and successful defenses on these grounds are rare.

The opposite speed limit is absolute speed limits, where a violation is illegal per se with no opportunity for a defense.

Speed limits in Texas, Utah, and Rhode Island are all prima facie. Arizona and some other states have a hybrid system where some or all speed limits are prima facie, but only up to a certain speed (usually between 80 and 100 mph). Most states simply have absolute speed limits.

Federal speed limit controls (55 mph)

Further information: National Maximum Speed Law

In response to the 1973 oil crisis, Congress enacted a National Maximum Speed Law that federally mandated that no speed limit may be higher than 55 mph. The law was widely disobeyed even after the national maximum was bumped to 65 mph in 1987 on certain roads. The law's 1995 repeal returned the choice of speed limit to each state.

State by state highlights

California

In California many speed limit signs are identified as "Maximum Speed", usually when the limit is 55 mph (90 km/h) or more. Rural Speed Limit on Interstate Highways Such as: I-5, I-8, I-10, I-15, I-40, I-80 and even CA-99 south of Madera, and Fresno, and U.S. 101 on the central coast carry the Maximum Speed of 70 mph (110 km/h), with supplementary signage stating "AUTOS WITH TRAILERS/TRUCKS 55 MAXIMUM." This is an accident of legal history. California's Basic Speed Law, of the California Vehicle Code, defines the maximum speed at which a car may travel as a reasonable and prudent speed, given road conditions. The numerical limit set by CalTrans engineers for "Speed Limit" signs, generally found on all non-controlled-access routes, is considered a presumptive maximum "reasonable and prudent" speed. However, it is technically allowable for a driver ticketed for exceeding this "Speed Limit" to present the argument that his or her speed was "reasonable and prudent" at the time the ticket was issued. While rare, some speeding tickets have been thrown out based on this provision. When the National Maximum Speed Limit was enacted, California was forced to create a new legal signage category, Maximum Speed, to indicate to drivers that the Basic Speed Law did not apply on these federally-funded highways; rather, it would be a violation to exceed the fixed maximum speed indicated on the sign, regardless of whether the driver's speed could be considered "reasonable and prudent".

Delaware

In Delaware, only two roads are posted with a 65 mph (100 km/h) speed limit: I-495 and Delaware Route 1. The remaining two Interstates, Interstate 95 and Interstate 295, along with all rural 4-lane non-Interstate highways, have 55 mph (90 km/h) speed limits. All rural 2-lane state-owned roads have 50 mph (80 km/h) speed limits, while all urban speed limits, regardless of location, is held at 25 mph (40 km/h) for 2-lane roads and up to 35 mph (55 km/h) for 4-lane roads. School zones are posted at 20 mph (30 km/h).

Indiana

In Indiana speed limits on Interstate highways are usually 70 mph (110 km/h) for cars and 65 mph (105 km/h) for trucks, except in urban areas, where it is generally 55 mph (90 km/h) in city centers and 65 mph (105 km/h) cars/60 mph (95 km/h) trucks in suburban areas. Prior to July 5, 2005, all Interstate highways were 65 mph and below.

Most non-Interstate highways are 55 mph (although some four-lane highways are 60 mph) and decrease to 45–30 mph (70–50 km/h) approaching urban areas, and within cities a speed limit of 20–30 mph (30–50 km/h) is not uncommon, though larger aterial roads within cities may reach as high as 45 mph (70 km/h).

File:Kansas Turnpike 80 MPH speed limit.jpg

Photo of 80 mph speed limit sign taken east of West Lawrence Interchange on what is now I-70 taken in 1957.

Kansas

The Kansas Turnpike used to have an 80 mph (130 km/h) speed limit. No Kansas road currently has a speed limit higher than 70 mph (110 km/h).

Montana

It is a myth that Montana had absolutely no rural highway speed limit from 1995 to 1999, whereas in fact there was only a brief period from December 1998 to June 1999 when this was the case, and even then a law prohibiting reckless driving, applicable to extreme speeds but less stringent than the previous "reasonable and prudent" standard, still applied.

In the years before 1974's 55 mph National Maximum Speed Limit and three years after its 1995 repeal, Montana had a non-numeric "reasonable and prudent" speed limit during the daytime on most rural roads. Montana Code Annotated (MCA) Section 61-8-303 said "A person . . . shall drive the vehicle . . . at a rate of speed no greater than is reasonable and proper under the conditions existing at the point of operation . . . so as not to unduly or unreasonably endanger the life, limb, property, or other rights of a person entitled to the use of the street or highway."

File:MONTANA-PR.png

Typical speed limit sign that one would see at the Montana border from December 1995 to June 1999.

Montana law also specified a few numeric limits: a night speed limit, usually 55 or 65 mph (90–100 km/h), depending on road type; 25 mph (40 km/h) in urban districts and 35 mph (60 km/h) in construction zones.

The phrase "reasonable and prudent" is found in the language of most state speed laws. This allows prosecution under non-ideal conditions such as rain or snow when the posted speed limit would be imprudently fast.

On March 10, 1996 , a Montana Patrolman issued a speed ticket to a driver traveling at 85 mph (140 km/h) on a lonely stretch of State Highway 200. The 50 year-old male driver was operating a 1996 Camaro with less than 10,000 miles (16,000 km) on the odometer. Although the officer gave no opinion as to what would have been a reasonable speed, the driver was convicted. The driver appealed all the way to the Montana Supreme Court. The Court reversed the conviction in case No. 97-486 on December 23, 1998; it held that a law requiring drivers to drive at a non-numerical "reasonable and proper" speed "is so vague that it violates the Due Process Clause ... of the Montana Constitution".

Due to this reversal, Montana scrambled to vote in a numerical limit as it technically had no speed limit whatsoever in the meantime. 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).

Montana law still contains a section that says "a person shall operate a vehicle in a careful and prudent manner and at a reduced rate of speed no greater than is reasonable and prudent under the conditions existing at the point of operation, taking into account the amount and character of traffic, visibility, weather, and roadway conditions." However, this is a standard clause that appears in other state traffic codes and has the practical effect of requiring a speed lower than the posted limit where a lower speed is necessary to maintain a reasonable and prudent road manner.

New Hampshire

The highest limit is a maximum lawful speed limit of 65 mph (100 km/h), which can be found on the Interstate System, the Central New Hampshire Turnpike and the Blue Star and Spaulding Turnpikes, in locations in which the highways are divided and have four or more lanes.

Provided that no hazard exists that requires lower speed, the speed of any vehicle not in excess of the limit is deemed to be prima facie lawful. The limit for "rural residential districts" and Class V highways outside the city or town compact is 35 mph. The limit for any "business or urban residence district" is 30 mph. School zones receive a 10 mph reduction in the limit 45 minutes before and after the beginning and end of a school day. The speed limit for a road work or construction area is 10 mph lower, but no more than 45 mph, than the posted limit when work is in progress. The speed limit for all other locations is 55 mph. The minimum limit that a speed can be set in a rural or urban district is 25 mph.

New York

The highest speed limit is 65 mph (100 km/h), which is found on most of the Thruway and other rural Interstate highways. The State Speed Limit (a blanket speed limit for rural roads) is 55 mph (90 km/h), which is also the highest a non-freeway highway may have. New York City (and some other urbanized areas) has a blanket speed limit of 30 mph (50 km/h) except where otherwise posted, however most freeways have a 50 mph (80 km/h) speed limit.

Governor Pataki signed legislation in September 2003 that enables NYSDOT and NYSTA to raise speed limits to 65 mph on its roads that meet established design and safety standards. This legislation became active in March 2004, and has been used on over 100 miles worth of highway. Prior to the new law, consent of the State Legislature was necessary to enact a 65 mph speed limit, a process that could take months or years.

Pennsylvania

In 1940, when the Pennsylvania Turnpike was opened between Irwin and Carlisle, the entire 110 mile highway did not have a posted speed limit, similar to that of the German Autobahns. In 1941, a speed limit of 70 mph (110 km/h) was established, only to be reduced to 35 mph (55 km/h) during the war years (1942–45). After WWII, the limit was raised to 70 mph on the four-lane sections, with the two-lane tunnels having 50 mph (80 km/h) for cars and 40 mph (65 km/h) for trucks. Prior to the 1974 federal speed limit law, all Interstates and the Turnpike had a 65 mph (105 km/h) speed limit on rural stretches and 60 mph (100 km/h) speed limit in urban areas.

In 1995, the state raised the speed limit on rural stretches of Interstate Highways and the Pennsylvania Turnpike system to 65 mph (105 km/h), with urban area having a 55 mph (90 km/h) limit. In 1997, PennDOT raised the speed limit to some rural non-Interstate highway bypasses to 65 mph (105 km/h). In 2005, with the change in the designation of "urban zones" in the state, the entire lengths of both the Pennsylvania Turnpike's east-west mainline and Northeast Extension were given 65 mph (105 km/h) limits, except at the tunnels and through the very winding 5.5 mile (9 km) eastern approach to the Allegheny Mountain Tunnel.

On non-freeway roads, speed limits are generally held at 55 mph (90 km/h) for rural two and four-lane roads, 45 mph (70 km/h) for urban four lane and state-owned two lane roads, 35 mph (55 km/h) for major roadways in residential areas, 25 mph (40 km/h) for most municipal residential streets, including main north–south and east–west roads in county seats, and 15 mph (25 km/h) for school zones.

South Dakota

Shortly after the December 1995 repeal of the 65/55 mph National Maximum Speed Limit, South Dakota raised its general rural speed limits to 75 mph on freeways and 65 mph on other roads. Almost a decade after posting the 75 mph limit, average speeds on South Dakotan rural freeways remain at or below the posted speed limit.

Texas

File:100 1816.JPG

Typical Texas rural speed limit sign. Note the black backgrounded 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 that does not prescribe a speed limit for each road type. 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

Texas statutorily allows the Texas Department of Transportation to post 75 mph (120 km/h) speed limits in counties with average populations of fewer than 15 people per square mile. The same statute also allows 80 mph (130 km/h) speed limits on I-10 and I-20 in certain rural west Texas counties with low population density. Daytime truck limits are capped at 70 mph, and nighttime speed limits remain 65 mph for all vehicles. (Nothing prohibits nighttime speed limits from being raised to 70 mph, but the Department has not elected to do so.)

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 bill also revised the eligibility for 75 mph speed limits: now eligible counties can have up to 15 persons per square mile. This did not substantially increase the miles of roadway eligible for higher limits, however.

Texas Legislator Pete Gallego unveiling a new 80 mph speed limit sign on Interstate 10 near Fort Stockton, Texas.

As of May 25, 2006, 4:11 PM, the Texas Transportation Commission has approved 80 mph speed limits, and signs are posted.

In a widely printed Associated Press story about the 80 mph speed limit, Texas is incorrectly reported as having legalized 75 mph limits in 1999. In fact, the bill that would have done this, HB 3328 by Pete Gallego, died in conference committee just before the Texas Legislature's session ended. This bill would have, in effect, set 75 mph as the statutory speed limit on any rural road numbered by the state or federal government, and it would have enacted—not simply allowed—an 80 mph speed limit on I-10 and I-20 in any county with fewer than 25,000 residents.

While Texas's 80 mph limit is higher than any limit authorized by another state, it is equivalent to the 130 km/h recommended speed on the Autobahn and the actual 130 km/h rural expressway speed limit in 13 other European countries. It is lower than the 150 km/h (95 mph) speed limit allowed on certain Italian roads and an experimental 160 km/h (100 mph) speed limit on an Austrian road.

Since Texas law allows 75 mph speed limits on any road numbered by the state or federal government, it is the only state with 75 mph limits on two-lane roads. Several west Texas two-lane roads carry 75 mph limits, including portions of US 90. No other state has a limit higher than 70 mph on any two-lane road.

85 mph limits

The legislation creating the Trans-Texas Corridor allows speed limits of up to 85 mph (140 km/h) on roads built under the program. However, no such roads have been built as of September 2006. The language of the statute does not prohibit the Texas Transportation Commission from raising the truck speed limit or the night speed limit on these roads.

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 lower this night speed limit or raise it to 70 mph, it in fact rarely does, so nearly every 70 mph or higher speed limit sign has an accompanying 65 mph night speed limit sign.

Texas law also prescribes a 55 mph night speed limit for trucks on farm to market roads, complementing the statutory 60 mph truck day limit on these roads. (This is a holdover from Texas's truck speed limits.)

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

Environmental speed limits

Texas

55 mph speed limit signs photographed on August 20 2002. These signs were on southbound I-45 at the north border of Montgomery County. Immediately before this point, the speed limit was 70 mph. These were part of an unpopular 55 mph speed limit cap in the Houston–Galveston area.

Same location as prior picture with the more recent 65 mph speed limit, photographed on August 4 2005.

Texas is the first state to lower speed limits for air quality reasons. 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 found that lower speed limits could bring the areas roughly 1.5% closer to compliance. However, followup studies found that the actual reduction is far less:

1. The emissions modeling software initially used, 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.
2. 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 10% (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.

In mid-2002, all speed limits in the Houston–Galveston area were capped at 55 mph (90 km/h). Facing immense opposition, poor compliance, and the finding 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 is only the typical pattern and 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.

Tennessee

Following the example of Texas, Tennessee has used environmental speed limits as part of pollution control efforts in Shelby, Hamilton, Knox, Jefferson, and Sevier counties, lowering the maximum truck speed limit to 55 mph (90 km/h) and the maximum car speed limit to 65 mph (100 km/h) within these counties.

Minimum speed limits

In addition to the legally defined maximum speed, minimum speeds may be posted. There is little evidence to suggest they are enforced.

Connecticut limited access divided highways have minimum speed of 40 mph (65 km/h), but they are not always posted.

Florida interstate highways have minimum speeds of 40 to 50 mph.

Illinois interstate highways are posted with both minimum and maximum speeds in many cases. The minimum speed is 45 mph.

Michigan freeways are usually posted with both minimum and maximum speeds. The minimum speed is currently 45 mph, but will be bumped to 55 mph effective 2006-11-09.

Mississippi has a minimum speed of 30 mph on Federal designated highways when no hazard exists. The minimum is 40 mph on Interstate highways and on 4 lane U.S. designated highways which have a posted maximum speed limit of 70 mph, but this minimum shall be posted. Mississippi has posted minimum speed limits (40 mph) on all rural Interstates. This minimum speed limit was already state law before the widespread posting.

Missouri and Minnesota interstate highways have minimum speed of 40 mph.

A small portion of Interstate 95 in New Hampshire has a minimum speed of 40 mph.

North Carolina has blanket minimum speed limits on interstate and primary highways only when signs are posted. The minimum is 40 mph if the maximum is 55 mph. The minimum is 45 mph is the maximum is at least 60 mph. These minimums do not apply to vehicles that are towing other vehicles.

In Tennessee, a minimum speed limit of 45 mph (70 km/h) is posted in metropolitan areas where the speed limit is 55 mph (90 km/h) or 65 mph (105 km/h).

Truck speed limits

Some jurisdictions set lower speed limits applicable only to large commercial vehicles like heavy trucks and buses. While they are called "truck speed limits", they generally do not apply to light trucks.

Theory

Since trucks are far heavier than other vehicles, they take longer to stop, are less adept at avoiding hazards, and have much greater crash energy. Therefore, it follows from basic physics that limiting truck speeds could reduce the severity and incidence of truck-related crashes.

However, the research record is mixed. A 1987 study finds that crash involvement significantly increases when trucks drive much slower than passenger vehicles, suggesting that the difference in speed between passenger vehicles and slower trucks could cause crashes that otherwise may not happen. Furthermore, in a review of available research, the Transportation Research Board, part of the United States National Research Council, states "[no] conclusive evidence could be found to support or reject the use of differential speed limits for passenger cars and heavy trucks" (page 11) and "a strong case cannot be made on empirical grounds in support of or in opposition to differential speed limits" (page 109) .

Two thirds of truck/passenger car crashes are the fault of the passenger vehicle.

Truck speed limit differentials

The following states have different statutory speed limits for cars and trucks.

State	Statutory car speed limit	Statutory truck speed limit
Arkansas	70	65
California	70	55
Idaho	75	65
Illinois	65	55
Indiana	70	65
Michigan	70	55 (increased to 60 effective 2006-11-09)
Montana	75	65
Ohio	65	65 on Ohio Turnpike, 55 on all other freeways.
Oregon	65	55
Texas	Except as described in the next row, Texas's statutory speed limit for all vehicles is 70 mph day/65 mph night. However, a separate statute allows the Texas Transportation Commission to raise daytime speed limits for passenger vehicles to 75 or 80 mph in certain counties. Such roads still retain a 70 mph day/65 mph night truck speed limit.
Texas (Farm-to-Market roads only)	70 day/65 night	60 day/55 night
Washington	70	60

Texas

Texas used to have a 60 mph (100 km/h) day/55 mph (90 km/h) night truck speed limit. This speed limit did not apply to busses or to trucks transporting United States Postal Service mail.

Whenever the speed limit on a road was above this threshold, separate truck speed limit signs were posted. These signs disappeared when all speed limits were capped at 55 mph (90 km/h) in 1974, but reappeared with the introduction of 65 mph (105 km/h) limits in 1986. Effective September 1, 1999, Texas repealed truck speed limits on all roads except farm to market and ranch to market roads.

Even after Texas repealed the truck speed limit, the Harris County Toll Road Authority erroneously retained the separate truck speed limits on its Interstate-class toll roads. The separate truck speed limits were removed with the 2002 adoption of the 55 mph environmental speed limit. The signs did not reappear when a 65 mph limit was imposed, but the truck speed limit sign posts are still standing as of January 2006.

2001 and 2003 statutes allowing 75 and 80 mph speed limits in certain areas of west and south Texas only apply to passenger vehicles. Truck speed limits remain 70 mph, so separate truck speed limit signs are slowly reappearing on these roads.

Due to the enormous unpopularity of a 55 mph speed limit cap that was imposed on the greater Houston area in 2002, the Texas Commission on Environmental Quality examined alternatives. Analysis suggested that the vast majority of emissions reductions from a 55 mph limit was from reduced heavy truck emissions. A proposed alternative was to restore passenger vehicle limits but retain a 55 mph truck speed limit. Concerns about safety problems and enforceability of such a large differential (up to 15 mph on many roads) scuttled that proposal, and a compromise plan, described above, was enacted that retained uniform, but still reduced, speed limits.

Louisiana

In August 2003, Governor Mike Foster announced speed and lane restrictions on trucks on the 18 mile (29 km) stretch of Interstate 10 known as the Atchafalaya Swamp Freeway. The restrictions lower the truck speed limit to 55 mph and restricts them to the right lane for the entire length of the elevated freeway.

Metric speed limits

File:US metric speed limit.png

Although not common, a speed limit may be defined in kilometers per hour (km/h) instead of miles per hour (mph), as the MUTCD states that "speed limits shown shall be in multiples of 10 km/h or 5 mph." If a speed limit is shown in km/h, the number is circumscribed and "km/h" is written below. The circle is added presumably to emulate the international speed limit sign, which is generally in km/h (e.g. mph in the UK).