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Letter from Pete Rahn

Executive Summary

Introduction

Keeping Drivers on the Road

Protecting Drivers Who
Leave the Road

What Does the Future Hold?

Putting It into Practice

Appendix: Resources

Keeping Drivers on the Road

Overview

Nearly 4 out of every 10 fatal motor vehicle crashes—over 16,000 a year—involve a single vehicle leaving its lane and/or the roadway, and there are more than twice as many fatal lane-departure crashes on rural roads than on urban roads. Some 42 percent of fatal lane departure crashes occur on curves (50 percent in rural areas), and the life-threatening events most likely to occur are rollovers (42 percent) and collisions with trees (25 percent).

 

 

 

Reducing the likelihood that a vehicle will leave the roadway through such roadway design strategies as flattening curves and installing shoulder rumble strips can prevent deaths and injuries resulting from roadway departure crashes. In addition, when a vehicle does encroach on the roadside, fatalities and injuries can be reduced if the agency minimizes the likelihood of the vehicle overturning or crashing into an object, or can reduce the severity of the crash.

Thus, the three key objectives in reducing the number of fatalities attributable to roadway departure crashes are:

1. Keep vehicles in their lanes and on the road.
2. Minimize the likelihood of crashing into an object or overturning if the vehicle travels off the shoulder.
3. Reduce the severity of the crash.

The first objective—keeping the vehicle in its lane and on the road—includes ways to communicate with the driver, but also includes strategies that target specific highway design features, such as shoulder drop-offs and pavements with low skid resistance.

The second objective—minimizing the likelihood of a crash when the vehicle leaves the roadway—includes strategies that focus on the roadside, especially that of higher-speed rural roads. Crash data analyses show that two-lane rural roadways are particularly vulnerable to roadway departure crashes because they do not have paved shoulders and wider, more forgiving clear zones.

Finally, the third objective–reducing the severity of the crash–includes vehicle design, restraint features, and the design of roadside features and geometry. While increased use of in-vehicle restraints would likely provide the greatest benefit, our emphasis is on roadway-related improvements.

 

Reducing Lane Departures

Preventing a vehicle from leaving its lane, or leaving the roadway entirely, is the first goal of highway safety engineers.  While this may not eliminate crashes with other vehicles or pedestrians, it can eliminate many fatalities that result when a vehicle strays from the lane onto the roadside or into oncoming traffic.

There are several low-cost, quick-implementation strategies that can help keep vehicles on the road and in their lanes:

Rumble Strips

Rumble strips add sound and vibration to the visual benefits of painted markings when traversed by vehicle tires. Rumble strips consist of either raised or grooved patterns that are installed perpendicular to the direction of travel.

Rumble strips can provide a drowsy, inattentive, or distracted driver with a clear warning that the vehicle has left the travel lane, and can allow increased reaction time before the vehicle leaves the road. They can be provided along the edge of the travelway, in the middle of the travel lane, or along the centerline of a two-way roadway.

Curve Delineation

Providing enhanced delineation of sharp curves, including markings, signing, and other high-visibility delineators, can also contribute to reduced lane-departure crashes. Delineation can provide drivers with a clear picture of the sharpness of the curve prior to entry and cause them to decrease their speed prior to entering the curve. Curve delineation can include shoulder treatments, such as chevrons, large arrow signs, or delineators on guardrails; improved curve warning signs, such as signs with flashing beacons; or innovative markings such as warning arrows on the pavement prior to the curve.

Enhanced Pavement Markings

Enhanced pavement markings can provide better “guidance” to drivers at locations where they might leave the roadway. These enhancements might include wider or higher contrast markings or raised pavement markers in spot locations, as compared with the standard pavement markings that would be used at other locations where the crash risk is lower.

Skid Resistance

Providing skid-resistant pavements will help with wheel traction, especially during wet weather events when friction forces are greatly reduced. In fact, a water film thickness of 0.002 inches reduces the tire pavement friction by 20 to 30 percent of the dry surface friction. Countermeasures to improve skid resistance include asphalt mixture (type and gradation of aggregate as well as asphalt content), pavement overlays on both concrete or asphalt pavements, and pavement grooving. Timely maintenance should also be conducted to reduce water build-up due to tire rutting, an inadequate crown, or poor shoulder maintenance.

In addition to general lane- and roadway-departure crashes, there are two potential situations that warrant special consideration: horizontal curves and head-on collisions.

Horizontal Curves

The average crash rate for horizontal curves is about three times that for straight segments. Statistical analyses show that 76 percent of curve-related fatal crashes involve single vehicles leaving the roadway and either striking fixed objects or overturning. Another 11 percent result in head-on crashes.

In addition to curve delineation strategies mentioned previously, other low-cost, quick-implementation strategies for reducing crashes in horizontal curves include the following:

Advance Warning of Changes in Horizontal Alignment

Advance warning lets a driver know that the horizontal alignment of the roadway is about to change and that the driver must alter the path, and possibly his or her speed, to negotiate the curve safely. Traditionally, advance warning is accomplished through the use of roadway signing, such as a “curve” sign and an advisory speed sign. Flashing beacons are often used to draw more attention to these signs.

Methods used on a more limited basis include warning messages placed on the pavement and rumble strips in advance of the very sharp curves. Additional non-traditional methods of advance warning involve pavement markings that attempt to cause a driver to reduce his/her speed. These markings include transverse lines with decreasing spacing or edge lines that give the appearance of a narrowing lane width.

Sight Distance

In horizontal curves, obstructions that limit the driver’s sight distance come in many shapes and forms. Physical features outside the traveled way, such as trees or bushes, guardrail or concrete barriers, and embankments, can limit the driver’s sight distance. As trees and other roadside vegetation mature, the sight distance at a horizontal curve may change. Efforts should be made to ensure that obstructions do not reduce the sight distance to less than the minimum stopping sight distance.

 

 

 

Additional, longer-term strategies for reducing crashes in horizontal curves include the following:

• Providing skid-resistant pavement surfaces
• Providing grooved pavement
• Providing lighting of the curve
• Providing a dynamic curve warning system
• Prohibiting/restricting trucks with very long semi-trailers on roads with horizontal curves that cannot accommodate truck off-tracking
• Widening the roadway
• Improving or restoring superelevation
• Installing automated anti-icing systems
• Designing safer slopes and ditches to prevent rollovers
• Modifying the horizontal alignment

 

 

Head-On Crashes

Most head-on crashes result from a motorist making an “unintentional” maneuver—the driver falls asleep, is distracted, or travels too fast in a curve. In fact, most head-on crashes are similar to roadway departure crashes—in both cases, the vehicle strays from its travel lane. About one in five non-intersection fatal crashes involves two vehicles crashing head-on. Of these, 75 percent occur on rural roads and 75 percent on undivided two-lane roads. For all roads, one-third of head-on crashes involve vehicles “negotiating a curve,” while two-thirds occurr on tangent sections.  

 

 

 

 

 

 

 

Low-cost, quick-implementation strategies for reducing head-on crashes include:

Centerline Rumble Strips

Centerline rumble strips are similar to shoulder rumble strips in that they alert drivers who inadvertently stray or encroach into opposing lanes. While this is a relatively new treatment, it has been implemented by several states, including Minnesota, Pennsylvania, Colorado, Delaware, Maryland, California, Washington, and Virginia.

Although there is no standard design, the rumble strip is generally wider than the center markings, extending into the travel lane by 5 inches to as much as 1½ feet. In some states, the strips are continuous along the centerline; in others, they alternate with a smooth gap. Since centerline rumble strips do not require changes in the overall cross section of the roadway, they are compatible with other strategies such as shoulder rumble strips and horizontal curve improvements.

Raised Centerline Delineation

Profiled Thermoplastic Stripes have been used for centerlines on two-lane roads by at least two states—California and Texas. Both states use this treatment for sections where passing is not permitted. While the treatment provides an audible/tactile effect, its principal benefit is the longer visibility distance provided at night, especially during wet conditions, when compared with standard pavement markings. However, as with standard raised pavement markers, this treatment should be limited to areas where there is little or no snow, as snow plow blades will easily scrape off the stripe.

“Buffer” Medians

An experimental strategy involves reallocating the existing two-lane cross section—narrowing lanes to encourage slower speeds while incorporating a narrow buffer median between opposing flows. For example, a high-speed rural two-lane roadway with a cross section of 12-foot lanes and 10-foot paved shoulders could be re-striped to provide narrower, 8-foot shoulders and slightly narrower, 11-foot lanes, with the difference forming a 6-foot flush median divider. The median could include milled-in centerline rumble strips to help prevent inadvertent crossings.

 

Additional, longer-term strategies for reducing head-on crashes include:

• Providing center two-way left-turn lanes for four- and two-lane roads
• Providing wider cross-sections on two-lane roads
• Using alternating passing lanes or four-lane cross-sections at key locations
• Installing median barriers for narrow-width medians on multi-lane roads

 

Protecting Drivers Who Leave the Road »

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