Successively Smaller Curves

Image Source: Google Earth

DEFINITION AND DESCRIPTION

A series of progressively sharper curves creates a self-enforcing roadway that encourages motorists to reduce speeds. The curves commonly take the form of reverse curves separated by a tangent. A broad-radius curve is followed by a moderate-radius curve. Actual speeds associated with these radii vary by roadway approach speed and deceleration needs.

Implementation Categories

CONSIDERATIONS

• Ideally, curve radii should not introduce speed differentials of more than 10 mph – 15 mph between successive curves.
• May be used in speed transition zones.
• Applicable to new construction and reconstruction projects.
• The length of the deceleration zone depends on the distance required to decelerate from the approach design speed to the target speed (See the AASHTO Green Book, Figure 2-34).
• Curve radii are calculated using the AASHTO Green Book (2018), Equation 3-8, with friction values from Figure 3-4.
• Often used on high-speed roundabout approaches and at freeway exit ramps.

APPLICATION

• Use in areas of speed transition zones in rural or suburban contexts on new construction or reconstruction projects.
• Use AASHTO criteria to design curvature. Guidance is also available in Section 5.2 of KYTC’s Roundabout Design Guidance.

example

Image Source: KYTC Roundabout Design Guidance

Complimentary Countermeasures

• Medians and Pedestrian Refuge Islands