Protecting Drivers Who Leave the Road
Once a driver has left the roadway, the goal of the highway safety engineer becomes two-fold:
- Minimize the likelihood of crashing into an object or overturning if the vehicle travels beyond the edge of the shoulder.
- Reduce the severity of the crash if an impact occurs.
Regardless of the reason that a vehicle leaves the roadway, a roadside free of fixed objects with stable, flattened slopes helps reduce crash severity. The “forgiving roadside” concept allows for the recovery of errant vehicles leaving the roadway and reduces the serious consequences of such an incident. Through decades of experience and research, the application of the forgiving roadside concept has been refined to the point where roadside design is an integral part of transportation design criteria. The basic design options for reducing roadside obstacles, in order of preference, are as follows:
- Remove the obstacle.
- Redesign the obstacle so it can be safely traversed.
- Relocate the obstacle where it is less likely to be struck.
- Reduce impact severity by using an appropriate breakaway device.
- Shield the obstacle with a longitudinal traffic barrier designed for redirection or use a crash cushion.
- Delineate the obstacle if the above alternatives are not appropriate.
The following are the most commonly encountered roadside features that should be addressed to minimize the likelihood of crashes.
Georgia's Pavement "Safety Edge"
Shoulder drop-offs can cause vehicles to overturn or lose control. One solution to this problem is to form a tapered transition at the edge of the paved surface called the “safety edge,” which helps errant vehicles maintain stability particularly on roadway re-entry. Based on the successful performance of a pilot project after one year in service, Georgia dot implemented the safety edge design on a statewide basis in 2005.
Shoulders
Roadway departure crashes can be reduced if a vehicle that enters the shoulder area is allowed to safely recover. Shoulder treatments that promote safe recovery include shoulder widening, shoulder paving, and the reduction of pavement edge drops—i.e., differences in lane pavement and shoulder surface heights, whether paved or not.
Shoulder treatments are often completed as a “package” during roadway resurfacing. These same shoulder treatments, particularly shoulder paving and correcting edge drops, can also reduce head-on crashes, as they enable the vehicle’s recovery to be made in a more controlled fashion and at a less sharp angle, thereby reducing the chance that the recovering vehicle will over-correct into the opposing lane.
Two of the most common fixed objects that are encountered along the roadside—and which lead to the highest number of fatalities—are trees and utility poles.
Trees
Trees are the most often struck fixed objects, involving over 3,000 fatal crashes per year—about 8 percent of all fatal crashes—and are most prevalent on rural roads. Of all fatal tree crashes, 90 percent occur on two-lane roads. Thus, rural two-lane roads should receive much of the focus in the development of any program to reduce tree-related highway fatalities.
Quick-Implementation Strategies for Reducing Crashes into Trees
- Implement planting guidelines to prevent placing trees in high-risk locations
- Follow mowing and vegetation control guidelines
- Delineate trees in high-risk locations
The issue of trees encompasses many DOT disciplines, and programs to address tree crashes should include planning, design, construction, and maintenance as they relate to roadway and roadside features. The general strategy for addressing trees includes preventing trees from growing in hazardous locations, and eliminating or reducing the severity of the potential obstacle.
New Jersey's Utility Pole Crash Pilot Program
New Jersey DOT has established a utility pole crash pilot program, which will start with 10 sites to be investigated for replacement of standard wood poles with breakaway poles.
Low-cost, quick-implementation strategies for reducing crashes into trees include:
- Developing and implementing planting guidelines to prevent placing trees in hazardous locations
- Developing and implementing mowing and vegetation control guidelines
- Delineating trees in high-risk locations
Other longer-term strategies include:
- Removing trees in high-risk locations
- Shielding motorists from striking trees
- Modifying the roadside clear zone in the vicinity of trees
Utility Poles
Each year, more than 1,000 fatal crashes are associated with utility poles. Because of the structural strength and small impact area of utility poles, these crashes tend to be severe.
Quick-Implementation Strategies for Reducing Crashes into Utility Poles
- Remove poles in high-crash locations
- Relocate poles in high-crash locations farther from the roadway and/or to less vulnerable locations
- Shield drivers from poles in high-crash locations
- Improve the drivers' ability to see poles in high-crash locations
- Revise policies to prevent placing or replacing poles within the recovery area
Utility poles should be addressed with a three-pronged attack:
- Treat specific utility poles in high-crash and high-risk spot locations.
- Prevent placing utility poles in high-risk locations.
- Treat multiple utility poles along a corridor to minimize the likelihood of crashing into a utility pole if a vehicle runs off the road.
Low-cost, quick-implementation strategies for reducing utility pole crashes include:
- Removing poles in high-crash locations.
- Relocating poles in high-crash locations farther from the roadway and/or to less vulnerable locations.
- Shielding drivers from poles in high-crash locations.
- Improving the drivers' ability to see poles in high-crash locations.
- Developing, revising, and implementing policies to prevent placing or replacing poles within the recovery area.
Other longer-term strategies include:
- Placing utilities underground.
- Relocating poles along the corridor farther from the roadway and/or to less vulnerable locations.
- Decreasing the number of poles along the corridor.
- Using breakaway devices.
- Applying traffic calming measures to reduce speeds on high-risk sections.
When a crash does occur, the final objective of the transportation agency is to help reduce its severity. This can be accomplished both by making roadside hardware more forgiving and by modifying side slopes to prevent rollovers.
Shoulder Treatments
Statistics indicate that the most harmful event in a non-intersection roadway departure crash is most likely to be an overturn. However, the rollover issue is complex in that roadway/roadside design is only one of many factors affecting rollovers. Additional factors include driver control, including speed and steering, and vehicle factors, such as the rollover propensity of SUVs and pickups.
Roadside design features most likely to affect rollover include the side slope, ditch design, the nature of the soil on the slope, and the design of roadside hardware that might lead to rollovers such as poorly designed guardrail ends. The best rollover prevention program is related to flattening and widening side slopes, particularly fill slopes.
Delaware's "Remove, Shield, Delineate" Program
Delaware DOT is partnering with the state utility companies to delineate all utility poles within the clear zone of the state highways. As part of their "remove, shield, delineate" program, all existing utility poles will be delineated by DelDOT forces, and all new poles will be delineated by the utility companies.
“An evaluation of safety measures in Iowa clearly shows that lane departure countermeasures provide the greatest return on investment of our limited safety funds. As a result, in addition to installing lane-departure countermeasures on reconstruction or new construction projects, we are also installing them on many of our resurfacing projects using 3R funds.”
Nancy Richardson, Director,
Iowa DOT
Roadside Safety Hardware
Some objects must be located near the traveled way, such as signs, lighting supports, culverts, and mail boxes. If an object cannot be removed from the clear zone or relocated farther from traffic, an alternative strategy is to shield the object or replace more substantial objects with breakaway devices or crash cushions. The AASHTO Roadside Design Guide includes design specifications, placement information, and crash test results for a large number of roadside hardware devices. The guide also includes criteria for use in determining which of the many alternative hardware types should be chosen for a specific application.
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