The following terms and abbreviations are commonly used in speed literature and discussions. Other references may use or define these terms somewhat differently than as defined below and used in this publication.

**15th percentile speed** – the speed at or below which 15 percent of vehicles travel. Also, see *Speed distribution*.

**85th percentile speed** – the speed at or below which 85 percent of vehicles travel. Also, see *Speed distribution*.

**A **– abbreviation for the algebraic difference in vertical alignment grades, in percent.

**AASHTO** – abbreviation for the American Association of State Highway and Transportation Officials.

**Advisory speed** – a speed below the speed limit that is recommended for a section of highway. The advisory speed is normally determined through an engineering study that considers highway design, operating characteristics and conditions. Advisory speeds are displayed on warning signs in speed values that are multiples of 5 mph. Advisory speeds cannot be enforced.

**Designated design speed** – the speed established as part of the geometric design process for a specific segment of roadway.

**e** – abbreviation for superelevation rate.

**f** – abbreviation for side friction factor.

**FHWA** - abbreviation for the Federal Highway Administration, an operating agency of the U. S. Department of Transportation.

**g** –force of gravity

**Green Book **– *A Policy on Geometric Design of Highways and Streets* (*1*) as published by AASHTO.

**IHSDM** – abbreviation for the Interactive Highway Safety Design Model, a collection of software tools that can be used to evaluate the safety and operational effects of geometric designs for two-lane rural highways.

**Inferred design speed** – the maximum speed for which all critical design-speed-related criteria are met at a particular location.

**K** – abbreviation for the rate of vertical curvature.

**L** – abbreviation for the length of vertical curve.

**Mean speed **- the summation of the instantaneous or spot-measured speeds at a specific location of vehicles divided by the number of vehicles observed. It is a common measure of central tendency.

**MUTCD **- abbreviation for the *Manual on Uniform Traffic Control Devices*.

**NHTSA **- abbreviation for the National Highway Traffic Safety Administration, an operating agency of the U. S. Department of Transportation.

**Operating speed** - the speeds at which vehicles are observed operating during free flow conditions. Free flow speeds are those observed from vehicles whose operations are unimpeded by traffic control devices (e.g., traffic signals) or by other vehicles in the traffic stream. The 85th percentile of the distribution of observed speeds is the most frequently used measure of the operating speed.

**Posted speed** – one of two speed limit types (statutory speed is other type); the maximum lawful vehicle speed for a particular location as displayed on a regulatory sign. Posted speeds are displayed on regulatory signs in speed values that are multiples of 5 mph.

**R** – abbreviation for horizontal curve radius.

**Side friction** – the lateral force developed between the tires and the roadway as a vehicle traverses a horizontal curve, expressed as a dimensionless coefficient of vertical force imposed by the vehicle’s weight.

**SSD** – abbreviation for the stopping sight distance.

**Sight distance** – the length along a roadway over which a driver has uninterrupted visibility – this is known as available sight distance. Different minimum sight distance design criteria exist for various operations and maneuvers, including stopping sight distance, passing sight distance and intersection sight distance.

**Speed deviation** – sometimes used to indicate *Standard deviation of speed* (see definition). Also, see *Speed distribution*.

**Speed distribution** – an arrangement of speed values showing their observed or theoretical frequency of occurrence.

**Speed limit** – the maximum lawful vehicle speed for a specific location. There are two types of speed limits, *posted speed* and *statutory speed*, definitions of each are provided.

**Speed zone** – a speed limit established on the basis of an engineering study for a particular section of road, for which the statutory speed limit is not appropriate.

**Standard deviation of speed** – a statistical measure (standard deviation) of the spread in values, applied to speeds. Also, see *Speed distribution*.

**Statutory speed** – one of two speed limit types (posted speed is other type). Numerical speed limits (e.g., 25 mph, 55 mph), established by state law that apply to various classes or categories of roads (e.g. rural expressways, residential streets, primary arterials, etc.) in the absence of posted speed limits.

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