Design Speed - the selected speed used to determine the various geometric design features of the roadway.
Operating Speed - the speeds at which vehicles are observed operating during free flow conditions. Free flow conditions mean that vehicles are unimpeded by other vehicles or by traffic control devices such as traffic signals.
85th Percentile Speed - the speed at or below which 85 percent of vehicles travel.
Target Speed - the highest operating speed at which vehicles should ideally operate on a roadway in a specific context.
Mean Speed - the summation of the measured speeds of vehicles at a specific location divided by the number of vehicles measured.
Speed Distribution - the arrangement of speed values showing their observed frequency of occurrence.
Posted Speed - the maximum lawful speed for a particular location as displayed on a regulatory sign.
Speed Limit - the maximum lawful vehicle speed for a specific location.
Statutory Speed - the numerical speed limits established by state law that apply to various classes or categories of roads in the absence of posted speed limits.
Speed Zone - the speed limit established on the basis of an engineering study for particular section of road, for which a statutory speed limit is not appropriate.
Advisory Speed - a recommended speed where the need to reduce speed below the speed limit is recommended due to a specific road condition.
The FHWA and ITE joint publication on Methods and Practices for Setting Speed Limits: An Informational Report is a comprehensive resource on speed limit basics for transportation professionals and provides the above industry definitions. The American Association of State Highway and Transportation Officials (AASHTO) publication A Policy on Geometric Design of Highways and Streets, 7th edition released in 2018, commonly referred to by the industry as the “green book,” has been adopted by the U.S. Department of Transportation as the design guide for federal roads, and state departments of transportation use it as the basis for state design manuals.
Transportation professionals involved in speed management use two terms when discussing the design and operation of a given road—the design speed and the operating speed. Design speed is used explicitly for determining minimum values for road design, such as horizontal curve radius and sight distance. The “green book” outlines means and methods that transportation professionals can use to select the appropriate design speed based on road type. Ultimately, design speed is selected by an agency with statutory authority to set speeds based on many factors. The selected design speed is then used to determine the various geometric design features, most notably the horizontal and vertical curvature. Other geometric design features such as cross-section elements, lane widths, shoulder width, the presence of curbs, etc., are determined based on the road function and safety. Most motorists will select a speed that they feel is reasonable and safe, influenced by the speed limit and design features of the road. Ideally, the speed a motorist feels is safe is the same as the designs speed selected.
Operating speed is a term used to describe the observed speed of a group of vehicles traveling on a section of road. A group of vehicles typically do not travel at the exact same speed; thus a speed study usually creates a speed distribution. With an engineering approach, a speed study is done for a specific road segment during a certain period of time for a specific sample set to determine mean speed and the speed distribution. From the speed distribution of a speed study, the 85th percentile can be determined from the cumulative distribution plot projecting horizontally from the 85th percentile level to the intersection of the plot and reading the speed at the horizontal axis. The 85th percentile speed may be determined to be the operating speed, may be considering in determining the safest design speed, or as a factor in setting a target speed. However, there are other factors that may need to be considered when determining the safest operating speed. Some transportation professionals are also considering other approaches, such as expert systems, safe system, and possibly a 50th percentile data point for safe urban streets. Ideally, the operating speed should be close to the speed limit.
A speed limit may be higher if vehicles can travel safely at a higher speed, or lower if there are other road users, such as bicyclists, that must also need a safe environment to travel. The FHWA Manual on Uniform Traffic Control Devices (MUTCD) recommends that speed studies be done when a road design changes, when traffic volumes fluctuate, or when there is an identified safety problem. The currently MUTCD states that the speed limit should be within 5 miles per hours (mph) plus or minus of the 85th percentile speed. The MUTCD does recognize that other road characteristics, such as alignment, parking practices, pedestrian activity, etc., are factors in setting safe speed limits. Additionally, a Vision Zero approach encourages setting speeds to also take into consideration other factors such as surrounding land use, the history of traffic crashes, injuries and fatalities, and existence of other permissible travel modes such as bicycling, walking, or riding transit.
Another term that some transportation professionals are considering relative to speed management and context sensitive design approaches is the concept of target speeds. The concept of target speed is outlined as the highest speed at which vehicles should operate on a thoroughfare in a specific context, consistent with the level of multimodal activity generated by adjacent land uses, to provide both mobility for motor vehicles and a safe environment for pedestrians, bicyclists, and public transit users. Transportation professionals looking for more information on context sensitive design and speed can review the ITE recommended practice on Designing Walkable Urban Thoroughfares: A Context Sensitive Approach or the ITE Design Factors to Control Speed fact sheet.
Transportation professionals should also consider special conditions, such as intersections, transition zones, work zones and school zones, bicycle and slow lanes, safe truck speeds, and possibly the need for variable speed limit systems when designing a road and determining speed limits. The below graphic shows an example of a transition speed zone from an area of a high rate of speed to a lower rate of speed due to change in road type.
The selection of the speed limit for any particular section of a road type is an exercise in weighing the objectives of safety and operational efficiency. The operational efficiency is measured by travel time and the safety level, measured by the incidence of crashes and resulting injuries and fatalities with consideration of the road function. Freeways and other high-level roads can accommodate higher speeds because of their design features that have little or no interaction with non-motorized user or adjacent development. Road types with lower functional classes, such as minor arterials, collector, and local roads provide for a mix of road users, requiring that safety take on a higher priority.
The FHWA Methods and Practices for Setting Speed Limits: An Informational Report describes three approaches for setting speed limits.
Engineering Approach - A two-step process where a base speed limit is set according to the 85th percentile speed, the design speed for the road, or other conditions. This base speed limit is adjusted according to traffic and infrastructure conditions such as pedestrian use, median presence, etc. Within the engineering approach there are two approaches: The Operating Speed Method is set within 5 mph of the 85th percentile speed determined from speed surveys and then appropriate changes plus or minus are made based on other considerations. Under the Road Risk Method, the level of roadside development and the function of a road are the primary determinants of the appropriate speed limit.
Expert System Approach - Speed limits are suggested by a computer program that uses knowledge and inference procedures that simulate the judgment and behavior of speed limit experts. Typically, this system contains a knowledge base containing accumulated knowledge and a set of rules for applying the knowledge to each particular situation. The FHWA developed USLIMITS2 is an expert system. USLIMITS2 is designed to determine speed limits in speed zones on all types of road, from rural two-lane segments to urban freeway segments. Based on input from the user, USLIMITS2 uses a decision algorithm to advise the user of the speed limit for the specific road section. Input into US LIMITS2 includes: surrounding development; access points; road function; road characteristics (e.g., divided or undivided, number of lanes, annual average daily traffic (AADT), roadside hazards, and section length) or freeway characteristics (e.g., number of interchanges, section length, and AADT); existing vehicle operating speeds (50th and 85th percentile); pedestrian activity; crash history; and special conditions (e.g., adverse alignment, transition zones, and parking). There is current industry discussion that an expert system, such as USLIMITS2, should be used to validate an engineering approach to speed limits.
Safe System Approach - The safe systems approach emphasizes that some degree of roadway user error will always occur, and that such errors should not result in a fatality or serious injury. With this approach, speed limits are set according to the crash types that are likely to occur, the impact forces that result, and the human body’s tolerance to withstand these forces. In the safe system approach, the primary criterion is the safety of all road users, including pedestrians and bicyclists that are more vulnerable to injury and death when hit by a vehicle. Consequently, this approach usually results in lower speed limits than those that would be determined by the engineering and expert system approaches. Tactics such as traffic calming, physical separation of roadway users, and treatments that enhance visibility of vulnerable users to give drivers greater reaction time are safe systems. A safe systems approach requires a holistic planning of the roads and interconnected factors provide for optimal safety. The safe systems approach is an ideal approach for many urban roads and to strengthen protection for vulnerable users.
The NCHRP report Design Speed, Operating Speed and Posted Speed Practices is also a helpful resource for transportation professionals looking for additional information on the principles of speed. Transportation professionals can learn more on speed limits via the FHWA Speed Limit Basics resource. The FHWA Speed Concepts: An Informational Guide, published in 2009, is helpful for transportation professionals to determine the most appropriate speed limits. Additionally, numerous Transportation Research Board (TRB) studies are currently being conducted to provide guidance on all factors to be considered in setting speed limits. A 1998 TRB Special Report on Managing Speed can also be helpful in setting and enforcing speeds. The National Committee on Uniform Traffic Control Devices (NCUTCD) is currently re-examining language in the MUTCD related to speed limit setting practices and has conducted a practitioner survey on this topic. The AAA Foundation for Traffic Safety also recently conducted a survey on the same subject. Additionally, the FHWA Roadway Safety Professional Capacity Building program technical assistance program can assist transportation professionals on speed limit methods and approaches.