Appendix 6– Pavement Edge Drop-offs
Differences in elevation between a travel lane and the adjacent lane, shoulder, or roadside
can be hazardous. Edge drop-offs can be caused by deterioration of the pavement or
adjacent ground, or by intentional removal or resurfacing of the adjacent surface during
maintenance or construction activities. Elevation differences are common occurrences in
work zones, as lanes and shoulders are resurfaced. The planning of work zones and
temporary traffic control should account for edge drop-offs if they will be present during
the work.
FHWA reports that an estimated 11,000 Americans suffer injuries and 160 die each year in
crashes related to pavement edge drop-offs. A pavement edge drop-off crash occurs when a
vehicle unintentionally crosses a drop-off and tries to get back into the proper lane. Contact
or scrubbing between the exposed pavement edge and the vehicle tire occurs as the driver
tries to steer back into the correct lane. This may result in over-steering and loss of control
of the vehicle. Sometimes the rear wheel may catch the edge of the pavement, swinging the
vehicle around and causing it to cross into the adjacent lane, where it may collide with or
sideswipe the oncoming vehicles, overturn, or run off the opposite side of the road. Vehicle
recovery becomes difficult and chances increase for loss of control of the vehicle to occur
when the elevation difference exceeds 3 to 4 inches.
Mitigation Measures
There are several practices and policies adopted by states to mitigate edge drop-off related
risks in work areas. Research indicates that the type of mitigation measure(s) selected
mainly depend on shape and depth of the vertical drop-off, speed limit or expected vehicle
speeds, longitudinal length of the drop-off, location of the elevation differential, the nature
of the adjacent surface, traffic volume, and duration of the work.
Section 6.2F of the Manual on Uniform Traffic Control Devices provides guidance on use of
signing to warn drivers of edge drop-offs. The LOW SHOULDER and SHOULDER DROP
OFF signs warn drivers of the elevation difference between the travel lane and the shoulder.
The LOW SHOULDER sign may be used when the drop-off is less than 3 inches, and the
SHOULDER DROP OFF sign should be used when the elevation difference is greater than 3
inches.
It is desirable that pavement edge drop-offs do not exceed 2 inches (AASHTO). For dropoffs less than 2 inches, UNEVEN LANES or SHOULDER DROP OFF warning signs may be
used. The distance between the warning signs should depend on the location of the drop-off
and the posted speed. This limit on the elevation difference varies from state to state. For
example, Missouri and Minnesota use 2 inches as the maximum drop-off for which only
warning signs are needed, whereas Ohio limits this drop-off to 1.5 inch.
One of the following mitigation measures may be used when drop-offs greater than 2 inches
are present:
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Wedge: A paved fillet or sloped wedge is placed between the edge of travel lane and
drop-off at a 45 degree angle or flatter slope. This provides a transition section that
allows the vehicle to return safely to the travel way.
Channelizing Devices: The drop-off may be shielded by placing channelizing devices
along the traffic side of the drop-off. The spacing of the devices may vary depending on
the speed of the roadway, traffic volumes and the differences in elevation between the
two surfaces.
Barricades: Temporary barrier rail may be used to shield the area. An adequate lateral
clearance or buffer between the barrier face and the travel way should be provided
along with the edge line delineation.
Warning signs should be used with all of bulleted items above.
References
American Association of State Highway and Transportation Officials. Roadside Design Guide.
Washington, DC. 2002.
Federal Highway Administration. The Safety Edge – Pavement Edge Treatment.
Federal Highway Administration. Manual on Uniform Traffic Control Devices.
Washington, DC. 2003.
Kannel, E., T.J. McDonald, M.M. O’Brien, V. Root. Traffic Control Strategies in Work Zones
with Edge Drop-Offs. CTRE Project 97-15. Iowa State University. August 2003.
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