Engineering Speed Limits

The management of speed through appropriate speed limits is an essential element of highway safety. Appropriate speed limits are a prerequisite for effective and sustainable speed management. In terms of traffic law, speed limits should reflect the maximum reasonable and safe speed for normal conditions. That is speed limits should be acceptable as reasonable by most drivers and separate high and low risk speed behavior. If lower speed limits are desired, then engineering and other measures should be implemented that reduce speeds to a level that would support a lower limit.

Manual on Uniform Traffic Control Devices Guidance
USLIMITS Expert Speed Zoning Advisor
Demonstration Projects on Setting and Enforcing Rational Speed Limits

Manual on Uniform Traffic Control Devices

The Manual on Uniform Traffic Control Devices (MUTCD) is the national standard for signing on all highways. Sections 2B.13-16 address regulatory speed limits and Section 2C addresses advisory speed signs. School zone speed limit signs are discussed in Section 7B.11 and work zone speed limits in Section 6C.

Return to top

USLIMITS

USLIMITS is a web-based expert speed zoning advisor to assist in setting appropriate and consistent speed limits. It provides an objective and consistent process for incorporating the various factors that may be considered in the decision making process according to the MUTCD but for which no guidance is provided. The speed zone expert system was adapted from similar expert systems used by most Australian state road authorities but modified to reflect elements of speed setting philosophy used in the US. A second-generation advisor based on US experts is under development as part of NCHRP Project 3-67.

The expert system recommends a speed limit for a section of road based on road function, roadside development, operating speeds, road characteristics and other factors required to determine appropriate speed limits in speed zones. The system also warns users of issues that might require further investigation and engineering judgment. USLIMITS provides a screen report and a more detailed print report. USLIMITS will be of particular use to small communities and agencies that lack experienced traffic engineers. For experience traffic engineers, it can provide a second opinion and increase confidence in speed zoning decisions. A user account is required to save projects and view the detailed speed zoning report, but anyone can trial USLIMITS by entering guest for the username and password.

Return to top

Demonstration Projects on Setting And Enforcing Rational Speed Limits

NHTSA and FHWA are jointly supporting efforts in seven states to demonstrate and evaluate an integrated "three E's" (engineering, enforcement, education) approach to the management of speed and crash risk. Rational speed limits are established on the basis of an engineering study of prevailing speed and other factors such as pedestrian activity and crash history. The 85th percentile speed is typically used as a starting point for setting a rational limit but it may be set as low as the average speed based on other factors. Once the speed limits are appropriately set and the judiciary informed, a program of strict enforcement with a low tolerance for speeds exceeding the limits is combined with PI&E explaining the purpose of the revised limits and the consequences for violators. Evaluation of program effectiveness is a critical element of the demonstrations. A list of our primary partners and the location of the demonstration roads are listed below.

Partner	Demonstration sites
Mississippi DOT	Major US arterial highway - Gulfport
Massachusetts Governors Highway Safety Bureau	Residential collectors - Natick
Connecticut State Police	Hebron
Tippecanoe County (IN) Highway Department	Two-lane county roads
City of Taylor (MI) Police Department	City streets & freeway connector
South Central Planning and Development Commission (LA)	Urban an rural roads - Terrebonne and Lafourche Parish
Virginia DOT	Freeway bypass - Martinsville and Altavista