5.12 Influence of Speed on Sight Distance

When operating speeds are much faster than design speed, increasing sight distance to account for higher vehicle speeds can be a good option. Table 5.8 summarizes how different aspects of a road’s design influences operating speed.

Table 5.8 Influence of Design Elements on Operating Speed
Design Element	Impact on Speed
Lane Width	Increasing lane width can translate into higher vehicle speeds.
Alignment	Curve radius significantly influences speed. Once a curve radius exceeds 2,625 ft, expect speeds like those observed on tangents.. Road characteristics that influence speed on tangents include tangent length, curve radii before and after a tangent section, cross section, grade, terrain, and sight distance. Posted speed is a better predictor of vehicle speed on urban arterial tangents than the highway tangents.
Pavement Surface	Vehicle speeds may increase slightly after pavement re-surfacing.
Roadside Elements	Drivers do not travel as fast when objects and other features are near the lane edge (e.g., parked vehicles, vegetation, pedestrians, bicyclists).

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