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FHWA Highway Safety Programs

Appendix A: Glossary

The following definitions are provided to aid in the understanding of setting speed limits. They may or may not coincide with terms and definitions found in related State statutes.

10 mph Pace: The 10 mph pace is the 10 mph range encompassing the greatest percentage of all the measured speeds in a spot speed study.

85th Percentile Speed: The 85th percentile speed is the speed at or below which 85 percent of the free-flowing vehicles travel.

Advisory Speed: Advisory speeds warn drivers to proceed at a speed lower than the speed limit due to geometrics, surface, sight distance, or other conditions.

Annual Average Daily Traffic: Commonly abbreviated as AADT, the total number of vehicles traversing a point or facility in one year divided by 365.

Average Speed: The average (or mean) speed is the most common measure of central tendency. Using data from a spot speed study, the average is calculated by summing all the measured speeds and dividing by the sample size.

Design Speed: The design speed is a selected speed used to determine the various geometric design features of the roadway.

Differential Speed Limit: A system that prescribes different maximum speed limits for different vehicle types or user groups. This is usually applied as one maximum speed limit for light passenger vehicles, and a lower maximum speed limit for trucks and heavy commercial vehicles.

Free-flow Speed: Free-flow speed is the speed a driver chooses when there are no influences from other vehicles, conspicuous enforcement, or environmental factors; in other words, this is the speed the driver finds comfortable based on the appearance of the road.

Injury Minimization Speed Limit: Also known as a speed limit for safe systems, it is a speed limit that is set so that the forces experienced by the human body in the event of a crash will not exceed biomechanical tolerances resulting in death or a severe personal injury.

Optimal Speed Limit: A speed limit that yields the minimum total cost to society, including vehicle operating costs, crash costs, travel time costs, and other societal costs.

Rational Speed Limit: A speed limit that is based on a formal, analytical review of traffic flow, roadway design, local development, and crash data. For existing roads, it uses the 85th percentile speed of free-flowing vehicles operating under normal traffic, weather, and roadway conditions as the speed limit, adjusted down by factors that can affect safety, such as road design features and roadside development and are not readily apparent to the motorist. The analysis also considers crash history and the influence of speed as a contributing factor. The 85th percentile speed is based on the premise that the vast majority of drivers will select a speed that is reasonable, safe, and prudent for a given road. Drivers who exceed the 90th percentile have a significantly higher risk of crashing.

Road Safety Audit: A formal safety performance examination of an existing or future road or intersection by an independent audit team.

Speed Dispersion: The speed dispersion refers to the normal spread in vehicle speeds observed in a study section.

Speed Limit, Absolute: An absolute speed limit is a numerical value, the exceeding of which is always in violation of the law, regardless of the conditions or hazards involved.

Speed Limits, Environment: An environmental speed limit is a speed limit created for the purpose of meeting federal air quality standards.19

Speed Limit, Posted: The posted speed limit is the value conveyed to the motorist on a black-on-white regulatory sign. Standard engineering practice is to post speed limits for freeways, arterials, and any roadway or street where speed zoning has altered the limit from the statutory value.

Speed Limit, Prima Facie: A prima facie speed limit is one above which drivers are presumed to be driving unlawfully. Nevertheless, if charged with a violation, drivers have the opportunity to demonstrate in court that their speed was safe for conditions at the time and not in violation of the speed limit, even though they may have exceeded the numerical limit.

Speed Limits, Statutory: Numerical speed limits specifically provided for under a State's traffic codes that apply to various classes or categories of roads (e.g., rural expressways, residential streets, primary arterials, etc.). State laws may or may not require that these limits be posted.15

Speed Zoning: Speed zoning is the process of performing and engineering a study and establishing a reasonable and safe speed limit for a section of roadway where the statutory speed limits given in the motor vehicle laws do not fit the road or traffic conditions at a specific location.

Speeding: The legal definition of speeding is exceeding the posted speed limit. In the road safety community, speeding is defined as exceeding the posted speed limit or speed too fast for conditions.

Test Run: A speed test run is performed by driving through a study area (potential speed zone) at a reasonable free-flow speed and collecting speed data, then using this data to confirm speed limits or speed data collected from other vehicles in the study area.