Editing Roadway Speed

The '''speed''' of a roadway can refer to one of four values in the context of transportation design.

== Speed Limit ==
The '''speed limit''' of a roadway is the maximum legal speed that motor vehicles may traverse the road.  Speed limits may differ by vehicle type, time of day, or immediate conditions (e.g. "speed limit 15 when children present").

== Operating Speed ==
'''Operating speed''', as defined by the ''Green Book''<ref>{{cite book|title=A Policy on Geometric Design of Highways and Streets|edition=7th|date=2018|publisher=American Association of State Highway and Transportation Officials|isbn=978-1-56051-676-7}}</ref>, is "the speed at which drivers are observed operating their vehicles during free-flow conditions".  The operating speed of a roadway is distinct from the ''design speed'' and ''speed limit''.

== Design Speed ==
The '''design speed''' is a value used by engineers when designing a roadway to help guide physical design characteristics.  In North Carolina, it is almost always set at 5 mph higher than the speed limit.  The ''Green Book'' uses the concept of design speed to assist in the choice of horizontal and vertical curvature, sight distances and barrier design among other things.

== Target Speed ==
'''Target speed''' is the operating speed that a roadway is intentionally designed to encourage, usually using traffic calming or other design tricks.  Target speed should ideally be equal to the speed limit.  For example, in a dense downtown, roadway designers may choose a target speed of 20 mph, and then use traffic calming strategies such as narrowing lanes, coordinating signals, speed tables, and visual complexity to ensure a satisfactory fraction of vehicles drive at or below that speed.

== References ==
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