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FHWA Home / Safety / Roadway Departure / Crash Types & Causes

Horizontal Curve Safety

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Horizontal curves are changes in the alignment or direction of the road, as opposed to vertical curves, which are a change in the slope. In 2008, more than 27 percent of fatal crashes occurred at horizontal curves; the vast majority (over 80 percent) were roadway departures.1 Due to the predominance of horizontal curves on typical rural roads, a higher percentage of fatal curve-related crashes occur on rural roads,2 particularly on two-lane roadways in rural areas. Fatality rates on rural roads are typically more than twice the rate than on urban roads, because of a number of infrastructure and non-infrastructure related issues.3

Photo of a curvy, relatively horizontal 4-lane road in the shape of an S as it winds along the outline of a mountainside.
Source: Oregon DOT

Minnesota Local Curve Safety Enhancement Program Example

In an 11-county region in southern Minnesota, local highway agencies reduced highway fatalities by implementing a Curve Safety Enhancement Program. By analyzing 20 years of crash data and then overlaying the crash data on a GIS layer, horizontal curve safety was found to be a primary crash concern. Thirty-two percent of crashes occurred on horizontal curves (composed of less than 10 percent of the highway system in the region) and 47 percent of severe roadway departure crashes occurred on horizontal curves. Minnesota DOT district engineers provided training and assistance in horizontal curve safety to local agencies within the 11-county region. County engineers inventoried and updated all existing curve signs and advisories. They also implemented an aggressive program for removal of roadside obstructions and enlisted the assistance of maintenance drivers in identifying obstructions. The Safety Edge was specified for on all pavement projects. As a result, highway fatalities in the 11-county region dropped from an average of three a year in 2002-2004, to 0.3 fatalities per year in 2005-2007.

Countermeasures

Most roadway departure countermeasures are effective when applied specifically at horizontal curves. Agencies should base selection of countermeasure strategies on sound engineering study or judgment. Some factors to consider include speed limit compliance, geometric features of the curve, sight distance, and traffic volume.

Photo of a rural two lane road with multiple horizontal curves.
Source: FHWA

For More Information

Low-Cost Treatments for Horizontal Curve Safety FHWA, December 2006. FHWA-SA-07-002.
http://safety.fhwa.dot.gov/roadway_dept/horicurves/fhwasa07002/

A Guide for Reducing Collisions on Horizontal Curves NCHRP Report 500, Vol. 7, 2004.
http://www.trb.org/Main/Public/Blurbs/154782.aspx

Joseph Cheung, P.E.
FHWA Office of Safety
Roadway Departure Team
joseph.cheung@dot.gov
202-366-6994
/



1 RDs and Curves_2005-2008, received from FHWA Office of Safety, July 2010. [ Return to note 1. ]

2 NHTSA Traffic Facts 2007 Data, Rural/Urban Comparison – DOT-HS-810-996. [ Return to note 2. ]

3 Implementing the High Risk Rural Roads Program, FHWA-SA-10-012, Federal Highway Administration, March 2010. [ Return to note 3. ]

Page last modified on June 24, 2011.
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