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Geometric Design: Glossary

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Acceleration Development Rate-- The rate at which the centripetal acceleration necessary to negotiate a horizontal curve is developed on the transition segment leading up to the curve.

Actual Sight Distance-- The sight distance provided by the highway as designed.

Brake Reaction Time-- The elapsed time between recognition of an object in roadway ahead and application of the brakes.

Braking Distance-- The distance traveled while braking to a complete stop.

Centripetal Force-- The force required to keep an object moving in a circular path. The centripetal force is always normal to the direction of the object.

Coefficient of Friction-- A dimensionless parameter that quantifies the resistance to sliding at the interface of two surfaces. 

Crest (vertical) Curve-- A curve that connects a segment of roadway with a segment of roadway that has a more negative grade. (uphill to level, uphill to downhill...)

Decision Sight Distance-- The sight distance that should be provided wherever drivers are forced to make decisions or are forced to cope with large amounts of information.  (Also see 'pre-maneuver time')

Design Speed-- The speed at which a vehicle should be able to traverse a roadway safely under favorable environmental conditions.

Grade (roadway)-- The slope of the roadway surface. Grade is expressed as the change in elevation per 100 feet of horizontal distance.

Horizontal Alignment-- The part of geometric design involved with designing the shape of the roadway within the horizontal plane.

Length (vertical curve)-- The horizontal distance from one end of the vertical curve to the other, or the horizontal distance between the VPC and the VPT.

Passing Sight Distance-- The sight distance required for drivers to feel comfortable about making a passing maneuver. 

Pre-Maneuver Time-- The time required for a driver to process information relative to a hazard, plot an avoidance course, and initiate the required avoidance maneuver.

Sag (vertical) Curve-- A vertical curve that connects a segment of roadway with a segment of roadway that has a more positive grade. (downhill to level, downhill to uphill...)

Side-Friction Factor-- The dimensionless factor used to describe the frictional resistance to slippage normal to the direction of travel.

Sight Distance-- The length of roadway ahead over which an object of a specific height is continuously visible to the driver.

Stopping Sight Distance, (Minimum)-- The distance required for a driver to react to a hazard in the roadway ahead and bring his/her vehicle to a complete stop. The sum of the distance required to stop the car and the distance traveled during the break reaction time.

Superelevation-- Inclined roadway cross-section that employs the weight of a vehicle in the generation of the necessary centripetal force for curve negotiation.

Superelevation Rate-- The slope of the roadway cross-section. For example, a 10 ft wide roadway with a superelevation rate of 10% would be 1 ft higher on one side than it is on the other.

Tangent Grade-- A grade that shares a common slope with the end of a vertical curve.

Vertical Alignment-- The portion of geometric design that deals with the shape of the roadway in the vertical plane.

Vertical Curve-- A parabolic curve used to provide a gradual change in grade between roadway segments with differing grades.