Improving the Safety of Horizontal Curves

Horizontal curves, where the road changes direction, are intended to make our roads safer and more efficient by allowing drivers to make gradual turns rather than sharp cuts. This is true for highways and rural two-lane roads and everything in between. The reality, however, is that horizontal curves are some of the most dangerous sections of any road. According to the U.S. Department of Transportation, “the average crash rate for horizontal curves is about three times that of other highways segments.” One-quarter of all highway fatalities happen on horizontal curves, even though they only make up a small percentage of total road miles.¹

The majority of these accidents and fatalities occur when drivers are speeding or not paying attention, causing them to miss the curve and leave the road. About three-quarters of curve-related fatal crashes involve single vehicles leaving the roadway.² These vehicles strike trees, utility poles, rocks or other fixed objects, or overturn.

Distracted driving. Speeding. Unpredictable road conditions. Throw in an unexpected change of direction, and you have a recipe for accidents. This challenge is so prominent, and the crash rates are so high, that the Federal Highway Administration (FHWA) recently determined that addressing horizontal curves is one of the most important things we can do to reduce accidents and improve road safety.

Recently, the FHWA identified 20 key treatments and strategies to address today’s road safety challenges. One of their proven countermeasures to improve road safety was “enhanced delineation and friction for horizontal curves.” Basically, this means making sure drivers can see approaching horizontal curves and have plenty of warning when a horizontal curve is coming up, and increasing the pavement friction around horizontal curves to help vehicles stay on the road.

High-visibility road markings and traffic signs around horizontal curves can catch drivers’ attention, alert them to an approaching curve and help them navigate the curve by encouraging them to reduce their speed or prepare to turn. Studies have shown that increasing the visibility of horizontal curves can reduce total crashes by 40%, reduce nighttime crashes by 57% and significantly reduce the severity of crashes when they do occur.³

The FHWA recommends treating horizontal curves with:
 

• Pavement markings
• Post-mounted delineation
• Larger road signs with enhanced retroreflectivity
• Dynamic advance curve warning signs

Each of these traffic safety devices is designed to alert drivers to approaching horizontal curves and help them safely drive around the curve. To get the most out of your road safety efforts, it’s important to understand key characteristics that increase the visibility and effectiveness of these traffic control devices.

When you’re choosing pavement markings and traffic signs for horizontal curves, these are key characteristics to keep in mind:
 

• Durability and Consistency: With consistent traffic and exposure to the elements, pavement markings on real-world roads can quickly become damaged or deteriorated. As a result, they can lose retroreflectivity and become more difficult to see. Pavement markings designed using more robust, durable materials are a worthwhile investment, as they’ll typically maintain more of their luminance, contrast and retroreflectivity over their lifespan.
• Retroreflectivity: Retroreflective optics are engineered to reflect light at the same angle from which the light originally came. This means that when light from a vehicle’s headlights hits a pavement marking with retroreflective optics, more light will be reflected back to the driver — improving the marking’s visibility.

¹ Source: Evaluation of the Sequential Dynamic Curve Warning System. (2015). U.S. Department of Transportation. Retrieved from https://www.fhwa.dot.gov/hfl/partnerships/tapco/final_report.pdf (PDF, 10.7 MB)
² Source: Evaluation of the Sequential Dynamic Curve Warning System. (2015). U.S. Department of Transportation. Retrieved from https://www.fhwa.dot.gov/hfl/partnerships/tapco/final_report.pdf (PDF, 10.7 MB)
³ Source: USDOT, Federal Highway Administration