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Planning Guidance for the Installation and Use of Technology Devices for Transportation Operations and Maintenance

ENTERPRISE Transportation Pooled Fund Study

Curve Warning System Guideline 1 - Rural Two-Lane Highway Curves

Purpose: To provide additional warnings beyond static advisory curve warning signs to warn drivers of actions required to reduce risks associated with rural two lane curves.

The planning guideline for curve warning systems are presented as one tool to assist agencies in selecting deployment locations, and are not a replacement for the planning or design process. Also, prior to deploying a Curve Warning System, ensure the basic requirements of the MUTCD (2009 Edition Chapter 2C. Warning Signs and Object Markers) have been met at the location of concern.

Engineering judgment should be used to assess potential implications that may result from installation of Curve Warning Systems. This planning guideline does not mandate the use of Curve Warning Systems.

1. The Radius of Curvature is less than 1,000 ft. Research conducted by Texas Transportation Institute found the risk of crashes on curves increases significantly when the radius of curvature is less than 1,000 ft. (1)

AND

2a. The horizontal curve is considered part of a visual trap (i.e. the beginning of the horizontal curve immediately follows a vertical curve and is hidden from the line of sight, or where the main road curves but a minor road (and sometimes utility lines) continue on the tangent). (2)

OR

2b. There is an observed pattern of vehicles entering the curve at speeds that are faster than safe speeds. For example, the 85th percentile speed exceeds the recommended or posted speed limit.

OR

2c. There are typically 2 or crashes on the curve each year. (3)

Note: if there has been at least one crash in the last year on the horizontal curve, a lower technology solution may be appropriate. Freeborn County (Minnesota) Safety Study identified high risk curves to have a combination of a visual trap, a low radius of curvature and at least one previous crash. (2)

AND

3. The highway is a 2-lane highway (1 lane each direction). (2)

AND

4a. The Speed Limit on the Highway is 55 MPH or greater. (2)

OR

4b. The Speed Differential (difference between the regulatory speed limit and the advisory speed limit) is 25 MPH or greater. (1)

References:

(1) Bonneson, J., M. Pratt, J. Miles, and P. Carlson. Development of Guidelines for Establishing Effective Curve Advisory. FHWA/TX-07/0-5439-1. Texas Department of Transportation, Austin, Texas. 2007.

(2) CH2M Hill. Freeborn County Road Safety Audit Review. 2008.

(3) Preston, Howard and Schoenecker, Ted. Potential Safety Effects of Dynamic Signing at Rural Horizontal Curves. Local Road Research Board, Minnesota Department of Transportation. 1999.