Countermeasure Assists in Road Re-entry
The Safety Edge is among the “Nine Proven Traffic Safety Countermeasures” being promoted by the Federal Highway Administration (FHWA) to help reduce traffic fatalities and injuries.
Safety Issue
During the traditional paving process, pavement edges have been typically formed in a vertical or near-vertical condition, and shoulder material is graded flush with the surface. But this only lasts so long and requires frequent maintenance, as with erosion and settlement, pavement edge drop-offs can develop.
When drivers inadvertently drop a tire off a pavement edge, they can have difficulty re-entering the road if the pavement edge is nearly vertical, especially if the height difference is significantly more than 2 inches. Figures 1 and 2 show near-vertical pavement edges with substantial drop-offs.
In attempting to move the tire back over a steep pavement edge, drivers may over-steer, possibly lose control of their vehicle, and cross into the path of oncoming traffic. Research indicates that pavement edges may have been a contributing factor in many rural run-off-the-
road crashes on paved roadways with unpaved shoulders. This type of crash was much more likely to include a fatality than rural crashes overall on similar roads.
Recommendation
A simple and cost-effective way to promote pavement edge safety is to adopt a standard specification for all resurfacing projects that requires the pavement edge to be tapered to 30 to 35 degrees from the horizontal. This is referred to as a “Safety Edge.”
This Safety Edge shape can be created by fitting resurfacing equipment with a paving device (see Figures 3 and 4) that extrudes and compacts the shape of the pavement edge to the desired 30- to 35-degree angle. The Safety Edge provides a tapered, compacted, and more stable pavement edge, making it easier for drivers to maneuver their vehicles safely back onto the paved surface from the unpaved shoulder.
Research in the early 1980s found a 45-degree pavement wedge effective in mitigating the severity of crashes involving pavement edge drop-offs. During more recent studies, sponsored by the FHWA, evaluation of wedge-paving techniques found it beneficial to flatten the wedge to a 30- to 35-degree angle (see Figure 5). Subsequent research has shown this design to be 50 percent more effective than the original 45-degree wedge.
After paving, the adjacent shoulder material should be graded flush with the top of the pavement. If erosion of the shoulder does occur, the Safety Edge provides a surface that can be more safely traversed.
This is a low cost technique that requires only a slight change in the paving equipment. For more information, go to http://safety.fhwa.dot.gov/roadway_dept/.
Editor’s Note: This article was prepared by Howard McCann, P.E., TEEX Transportation Training Director, from material provided by the Federal Highway Administration. Originally printed in TEEX’s Lone Star Roads, 2010, Issue 4. Reprinted with permission.