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Federal Highway Administration Research and Technology
Coordinating, Developing, and Delivering Highway Transportation Innovations

This report is an archived publication and may contain dated technical, contact, and link information
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Publication Number:  FHWA-HRT-17-098     Date:  January 2018
Publication Number: FHWA-HRT-17-098
Date: January 2018


Self-Enforcing Roadways: A Guidance Report


85th-percentile speed Speed at which 85 percent of the motor vehicles are traveling at or below.
Braking distance (db) Distance traveled by a vehicle once the brakes are applied until the vehicle completely stops (ft (m)).
Designated design speed (design speed) Speed used to establish certain geometric elements of a roadway (mph (km/h)).
Desired speed Speed drivers wish to travel based on the alignment characteristics of a roadway, including topography, cross section, adjacent land use, and traffic volumes.
Green Book A Policy on the Geometric Design of Highways and Streets. (AASHTO 2011)
Horizontal sight line offset (HSO) Lateral offset along a horizontal curve measured from the centerline of the innermost travel lane to a sight line obstruction (ft (m)).
Inferred design speed Speed at which all critical geometric design-related criteria are met at a certain point along a roadway (mph (km/h)).
Lane width Portion of roadway designated for motor vehicle use (ft (m)).
Length of vertical curvature (L) Distance from the point of vertical curve (PVC) to the point of vertical tangency (PVT) (ft (m)).
Maximum side friction factor (fmax) Maximum side friction demand used in horizontal curve design. The maximum side friction factor is based on driver comfort levels (i.e., tolerance for lateral acceleration) and is also referred to as the limiting side friction factor. (Torbic et al. 2014)
Maximum superelevation (emax) Maximum cross slope of the roadway cross section within the limits of a horizontal curve. This value ranges from 4 to 12 percent, depending on climatic conditions, area type, terrain, and the frequency of very slow-moving vehicles in the traffic stream.
Operating speed Speed at which motor vehicles are observed traveling on a given roadway during free-flow conditions (mph (km/h)).
Posted speed limit Maximum speed that can be legally driven on a given roadway. The speed limit is typically posted on regulatory signs (mph (km/h)).
Radius of curvature (R) Provides for transition between two tangent segments of roadway (ft (m)).
Rate of vertical curvature (K) Horizontal distance necessary to affect a 1-percent change in the vertical gradient of the roadway, which is defined by the length of vertical curvature (L) divided by the algebraic difference in grades (A).
Shoulder width Width of the area parallel to the travel lane and part of the roadway; the shoulder can be either paved or unpaved (ft (m)).
Stopping sight distance (SSD) Distance required for a driver to perceive, react, and brake to a stop (ft (m)).
Target speed Anticipated or desired operating speed of a roadway or section of roadway.
USLIMITS2 Web-based tool developed by the FHWA used for guidance regarding the setting of appropriate posted speed limits for all road types. (FHWA 2016b)
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