Define the concept of geometric design speed in highway design and its relationship to stopping distance.

• A. A target speed used to size alignment features; stopping distance increases with design speed, influencing sight distance and radii.
• B. The maximum speed allowed on the road.
• C. The average speed of traffic during peak hours.
• D. The minimum speed required for safe operation.

Answer: (A)

Geometric design speed is the target speed engineers use to size the road’s horizontal and vertical alignment so that the roadway can be safely and comfortably traveled at that speed. The way it ties to stopping distance is direct: stopping distance grows with speed because both the perception-reaction distance and the braking distance increase as speed rises. Therefore, the design speed sets the required stopping sight distance and influences how sharp curves and other alignment features can be, since the sight distance and radii must be sufficient for safe stopping at that speed. It’s a planning/design parameter, not the posted speed limit, nor the typical traffic speed, nor a minimum speed.