6.8 Countermeasures for Improving Steering and Vehicle Control Through Curves

Motorists retain better control of vehicles when curves conform with their expectations and typical behaviors. Several design strategies promote safer alignments (Table 6.2).

Table 6.2 Design Strategies to Improve Steering Control Through Curves
Feature	Guidance
Curvature	Minimize the use of maximum allowable curvature for a given design speed.
Spirals	Use spiral transition curves whenever possible, especially on roads with design speeds ≥ 60 mph. The length of a spiral curve should equal the distance traveled during steering time (2 – 3 seconds, depending on the radius) Recommended curve radius for two-lane roads with a speed limit of 50 mph is 393 – 755 ft. Clothoid parameters should be 0.33 b – .05 R.
Reverse Curves	Do not use tangent sections in reverse curves when the distance between the first curve’s exit and the second curve’s entrance is short enough to establish a curved path through the tangent. This is 262 ft or less for two-lane highways and 443 ft for freeways.
Superelevation	Design the superelevation to result in zero lateral acceleration through a curve at design speed.
Design Consistency	Avoid the use of sharp, isolated curves. Maintain consistency in superelevation, road width, and other curve features.

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Chris VanDyke

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chrisvandyke@uky.edu