Setting speed limits in the United States has always been a responsibility of State and local governments. The unrestricted freedom to exercise that authority was interrupted by the Federal Government during World War II, and more recently with the National Maximum Speed Limit of 55 mph (90 km/h). The National Maximum Speed Limit was repealed in 1995.

Every State has a basic speed statute requiring drivers to operate their vehicles at a speed that is reasonable and prudent for conditions. This basic rule is contained in the Uniform Vehicle Code (UVC), which provides a model set of motor vehicle laws to encourage uniformity in State traffic regulation. State statutes authorize maximum speed limits that may vary by highway type (e.g., interstate highways) or location (e.g., urban district).¹¹

The UVC is a set of model traffic laws that was originally developed by the National Committee on Uniform Traffic Laws and Ordinances (NCUTLO), a now defunct, private, non-profit organization. The NCUTLO's members were mainly State governments and some related organizations. The extent to which the code is used varies by State. The UVC and most State motor vehicle laws include a basic speed law with wording similar to the following: No person shall drive a vehicle at a speed greater than is reasonable and prudent under the conditions and having regard for the weather, visibility, traffic, and the surface and width of the roadway.¹¹

Section 11-803 of the UVC recommends States establish speed zones upon the basis of an engineering and traffic investigation. Section 11-804 outlines recommended practices on how local authorities may alter maximum limits.¹²

Article VIII—Speed Restrictions

11-801—Basic rule

No person shall drive a vehicle at a speed greater than is reasonable and prudent under the conditions and having regard to the actual and potential hazards then existing. Consistent with the foregoing, every person shall drive at a safe and appropriate speed when approaching and crossing an intersection or railroad grade crossing, when approaching and going around a curve, when approaching a hill crest, when traveling upon any narrow or winding roadway, and when special hazards exist with respect to pedestrians or other traffic or by reason of weather or highway conditions. (Revised, 1968)

Uniform Vehicle Code and Model Traffic Ordinance, 2000, National Committee on Uniform Traffic Laws and Ordinances, Evanston, Illinois.

Types of Speed Limits

Speed limits may be classified as default/statutory regulations, or speed zoning regulations established on the basis of engineering studies. In all cases, a speed limit must be legislated (i.e., established by legislative authority).

Statutory Speed Limits

Statutory limits are based on the concept that uniform categories of highways can operate safely at certain maximum speeds under ideal conditions. State motor vehicle laws specify speed limits on specific categories of streets and highways. For example, a vehicle code might limit speeds to 25 mph (40 km/h) in residential areas, 30 mph (50 km/h) in business districts, and 55 mph (90 km/h) on all other roads. Generally, statutory limits apply throughout a political jurisdiction.¹¹ Table 1 contains examples of statutory limits for three States and for the Uniform Vehicle Code.

Table 1. Examples of Speed Limit Statutes

Statutory speed limits allow for speed limits to be in effect even when it is not practical to post them.

There are two types of statutory speed limits: (a) absolute limits and (b) prima facie limits. The principle difference between the two types is whether someone who is charged with driving over the speed limit can defend her/his actions. An absolute speed limit is a limit above which it is unlawful to drive regardless of roadway conditions, the amount of traffic, or other influencing factors. There is no recourse to contend a charge. A prima facie speed limit is one above which drivers are presumed to be driving unlawfully but, if charged with a violation, they may contend that their speed was safe for conditions existing on the roadway at that time. And, therefore, that they are not guilty of a speed limit violation.

Prima facie limits provide greater flexibility to drivers to determine an appropriate speed for conditions and place a greater burden of proof on the enforcement community that a violation has occurred.

Approximately two-thirds of the States have absolute speed limits.¹¹

Speed Zones

Where statutory limits do not fit specific road, traffic, or land uses conditions, most road authorities have the power to establish speed zones to reflect the safe maximum reasonable speed. These alternative speed limits may be higher or lower than those prescribed by the UVC or the statutory limits of the jurisdiction. Alternative maximum legal speed limits are established by legislating the speed zone, typically founded on the basis of an engineering study, and becoming effective when the limits are posted and properly recorded.¹¹ Agencies process resolutions, traffic control orders, or other formal documents to properly record the legal speed limit. An example of a Traffic Control Order is shown in Appendix B.

To encourage compliance and effectively manage risk, many agencies set speed limits to reflect the "reasonable and prudent" behavior of the majority of motorists acting in an appropriate manner. This encourages drivers to obey the posted speed limit and travel at a reasonable speed. It also targets limited enforcement resources at the occasional violator who disproportionately contributes to crash risk. The concept of a rational speed limit involves a formal engineering review, during which drivers' free-flowing speeds are observed. The assumption is that by reflecting actual driver speeds, most people will consider the speed limit appropriate. Such speed limits are desirable because they encourage public compliance, reduce speed differences among drivers, and offer a defensible enforcement tool.