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30.2 Reducing Vehicle Speed Project | Safe Design

30.2 Reducing Vehicle Speeds Approaching Roundabouts

Lowering the speed of vehicles approaching roundabouts improves yielding behavior, heightens motorist awareness of pedestrians and bicyclists, and mitigates crash severity. Designers can leverage several strategies to lower vehicle speeds on roundabout approaches (Table 30.1).

Table 30.1 Design Guidance to Lower Roundabout Approach Speeds
Design Element	Strategy
Sight Distance	Provide only the minimum required sight distance. Along with lowering approach speeds, this minimizes speed differentials. Excessive sight distance can increase vehicle speeds and compromise the safety of all users.
Lane Width	Narrowing the approach and exit lane widths reduces vehicle speeds.
Vehicle Path Radius at Entry and Exit	Reduce the vehicle path radius at the roundabout entry and exit. This reduces vehicle approach and exit speeds.
Inscribed Circle Diameter	Reduce the inscribed circle diameter to lower vehicle speeds. Verify the diameter is large enough to accommodate the design vehicle.
Transverse Rumble Strips	Installing transverse rumble strips on approaches can reduce speed but may increase speed differentials.
Guide Posts	Install guide posts (using even or uneven spacing) next to the vehicle path entering and within a roundabout.
Center Island Landscaping	Plant trees and vegetation to reduce both vehicle speeds and speed differentials.

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