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Roadway Departure Safety (Part 1 of 3)

Edited by Airton G. Kohls and Matt Cate (Source: FHWA Office of Safety)

While less than 20 percent of the U.S. population lives in rural areas, NHTSA data shows that more than half of roadway fatalities occur on rural roadways.

A roadway departure (RwD) crash is defined as a crash which occurs after a vehicle crosses an edge line or a center line, or otherwise leaves the traveled way. Roadway departure crashes encompass a variety of crash scenarios, including collisions between vehicles traveling in opposite directions on the same roadway, vehicles striking terrain and drainage features, and collisions with fixed objects such as trees and utility poles.

This article is the first of three articles we will be using in 2019 to highlight resources that can help your agency reduce RwD crashes. TTAP can also help you with training on several of these resources. Please check our training calendar at http://ttap.utk.edu/training/course-calendar.php.

While less than 20 percent of the U.S. population lives in rural areas, NHTSA data shows that more than half of roadway fatalities occur on rural roads. Further, two-thirds of these rural fatalities involved a roadway departure (RwD). This means that not only are deaths on rural roadways severely overrepresented, so are fatalities and serious injuries related to RwD crashes.

As mentioned on a previous article of RoadTalk, to address this considerable safety issue on the Nation's rural road systems, FHWA has selected "Reducing Rural Roadway Departures" as 1 of the 10 innovations that are the focus of round 5 (2019-2020) of the Every Day Counts (EDC-5) initiative.

To effectively prevent RwD crashes and fatalities, FHWA's efforts are guided by the Strategic Approach & Plan that follows a clear framework prioritizing efforts to:

  1. Keep vehicles on the roadway, in their appropriate directional lane
  2. Reduce the potential for crashes when vehicles do leave the roadway or cross into opposing traffic lanes
  3. Minimize the severity of crashes that do occur

Additionally, this approach aims to increase awareness of current safety needs, future priorities, and collaboration opportunities with other program areas to enhance the success of focused safety efforts.

Countermeasures to Keep Vehicles on Roadway

The most cost-effective solution to reduce the number of crashes, injuries, and fatalities due to roadway departure is to strategically focus efforts on implementing countermeasures that will keep the vehicles on roadway.

Improving pavement friction, alerting drivers with rumble strips, enhancing delineation along horizontal curves, and improving nighttime visibility are effective practices that FHWA encourages agencies to explore in order to keep vehicles on the roadway.

 

Pavement Friction

A roadway must have an appropriate level of pavement friction to ensure that drivers are able to keep their vehicles safely in the lane. Poor pavement conditions, especially wet pavement, have been identified as one of the major contributing factors in roadway departure crashes. When a pavement surface is wet, the level of pavement friction is reduced, and this may lead to skidding or hydroplaning.

Vehicles have different friction demands depending on the characteristics of the roadway. For example, a vehicle traversing a horizontal curve requires a greater level of friction than vehicles on a straight section. Common locations that require higher friction values are horizontal curves, steep grades, or intersection approaches. As a result of the increased friction demand, the roadway surface at these locations often becomes prematurely polished, reducing the pavement friction and contributing to higher crash rates. Excessive speeds or distracted driving can also contribute to the crash rates in areas where friction demand is high.

Pavement friction is critical for changing vehicle direction and ensuring the vehicle remains in its lane. Traditional friction courses or high friction surface treatments should be considered for curves with numerous wet weather crashes or severe curves with higher operating speeds.

For additional information on pavement friction, please go to: https://safety.fhwa.dot.gov/roadway_dept/pavement_friction/

 

Rumble Strips and Rumble Stripes

Rumble strips are an effective countermeasure for reducing roadway departure crashes. The noise and vibration produced by rumble strips alert drivers when they leave the traveled way. Rumble stripes is the term used for rumble strips painted with a retroreflective coating to increase the visibility of the pavement edge at night and during inclement weather conditions.

Research has shown that installing rumble strips can reduce severe crashes. The following tables illustrate the safety effectiveness of center line and shoulder rumble strips. This information, along with additional statistics, is contained in NCHRP 641: Guidance for the Design and Application of Shoulder and Center Line Rumble Strips, 2009.

For additional information on rumble strips and rumble stripes, please go to: https://safety.fhwa.dot.gov/roadway_dept/pavement/rumble_strips/

Center line Rumble Strip - Reduction in crash frequency from before to after rumble strip implementation for head-on and opposite direction sideswipe fatal and injury collisions
 Percent reduction in crash frequency from before to after rumble strip implementationStandard Error
Rural two-lane roads45%6%
Urban two-lane roads64%27%
Excerpt from Table 67 of NCHRP Report 641.

Shoulder Rumble Strip - Reduction in crash frequency from before to after rumble strip implementation for single-vehicle run-off-road fatal and injury crashes
 Percent reduction in crash frequency from before to after rumble strip implementationStandard Error
Rural two-lane roads36%10%
Urban two-lane roads17%7%
Excerpt from Table 28 of NCHRP Report 641.

 

Horizontal Curve Safety

Horizontal curves are those that change the alignment or direction of the road (as opposed to vertical curves, which change the slope). More than 25 percent of fatal crashes are associated with a horizontal curve, and the vast majority of these crashes are roadway departures. The average crash rate for horizontal curves is about three times that of other types of highway segments. About three-quarters of curve-related fatal crashes involve single vehicles leaving the roadway and striking trees, utility poles, rocks, or other fixed objects-or overturning.

Most roadway departure countermeasures are effective when applied specifically at horizontal curves. A focus on horizontal curves can prove to be a cost-effective approach to reducing roadway departure crashes. Many of these countermeasures are low-cost and can be installed at prioritized horizontal curves to address safety issues.

 

Please go to the link below and check this new available resource (FHWA's Indoor Simulator and Field Study Evaluation of Sequential Flashing Chevron Signs on Two-lane Rural Highways): https://safety.fhwa.dot.gov/roadway_dept/countermeasures/horicurves/docs/fhwasa18075.pdf

Nightime Visibility

About half of traffic fatalities occur at night, although only about one quarter of travel occurs after dark. While intoxication and fatigue contribute to the high rate of nighttime crashes, nighttime driving is inherently challenging due to decreased visibility. In dark conditions, retroreflective pavement markings and signs delineate the roadway alignment and improve the visibility of decision points such as intersections. Adequately maintained retroreflective signs improve nighttime highway navigation and reduce the risk of crashes by bouncing light from vehicle headlights back toward the vehicle and the driver's eyes, making the signs appear brighter and easier to see and read. Because the retroreflective properties of traffic control devices deteriorate over time, highway agencies need to actively manage the maintenance of signs in order to ensure that they remain clearly visible at night.

 

For additional information on nighttime visibility, please go to: https://safety.fhwa.dot.gov/roadway_dept/night_visib/


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