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. These commonly take the form of reverse curves separated by a tangent, where 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

• Successive curves should not introduce speed differentials of more than 10 mph – 15 mph between each curve.
• May be used in speed transition zones.
• Applicable to new construction and reconstruction projects.
• The required 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 side-friction factors from Figure 3-4.
• Often used on high-speed roundabout approaches, freeway exit ramps, and speed transitions on rural highways.
• Ensure adequate sight distance is maintained through the sequence of curves.

APPLICATION

• Use in areas of speed transition zones in rural or suburban contexts on new construction or reconstruction projects.
• Effective at roundabout approaches and exit ramps where speed reduction is critical.
• Apply AASHTO criteria to design horizontal curvature. Guidance is also available in Section 5.2 of KYTC’s Roundabout Design Guidance.

example

Image Source: KYTC Roundabout Design Guidance

complementary Countermeasures

• Medians and Pedestrian Refuge Islands