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Free Flow Speed (FFS) - Definition - Factors Affecting FFS

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Definition

Free-flow-speed of a vehicle is defined as: The desired speed of drivers in low volume conditions and in the absence of traffic control devices. The mean speed of passenger cars that can be maintained in low to moderate flow rates on a uniform freeway segment under prevailing roadway and traffic conditions.

Factors affecting free flow speed

Factors affecting free-flow speed of a vehicle are:

  1. Width
  2. Lateral clearance
  3. Number of lanes
  4. Interchange density
  5. Geometric design
  6. Weather (The amount of reduction in free-flow speed is directly related to the severity of the weather event.)
  7. Visibility

Based on a study of two-lane rural highways, Lamm, Choueiri, and Mailaender found that drivers do not adjust their speeds very much under light rain or wet pavement, but they do reduce speeds when visibility becomes obstructed, such as during a heavy rain.

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