In this article the highway engineering experts at Robson Forensic discuss how pavement drop-offs are created, why they are potentially dangerous, and what highway engineers should be doing to reduce drivers’ exposure to this potentially dangerous condition.
A pavement or shoulder edge drop-off is a condition where there is a significant elevation change (uneven pavement) from one travel lane to another, or between a travel lane and the adjacent shoulder. This condition is known to be hazardous to motorists, particularly motorcycles, compact cars, and vehicles pulling trailers.
Conventional paving techniques used on highway reconstruction projects often result in vertical or nearly vertical pavement edges between completed surface work and the uncompleted surfaces in adjacent travel lanes. The magnitude of the drop-off is roughly equal to the thickness of the top course of asphalt on the finished lane (which sits above the unpaved base course in the adjacent travel lane); however, irregularities in the paved top course of asphalt can cause the height of the drop-off to be higher, sometimes significantly so. The face of the drop-off is usually vertical, which can cause safety concerns for drivers that are exposed to them.
Shoulder Edge Drop-Offs
Shoulder edge drop-offs can occur when a road is resurfaced without adequately raising the height of the shoulder. They can also occur when a road and shoulder are resurfaced where the ground abutting the shoulder (roadside) is not raised to the same height of the shoulder. This not only creates a drop-off but leaves the shoulder unsupported laterally, which can lead to edge cracking and shoulder failure. On roads with unpaved shoulders (earth, grass, stone, etc.), drop-offs will often occur in areas where vehicles tend to leave the paved surface, such as adjacent to horizontal curves, near roadside mailboxes, and at turnarounds/unpaved pull-outs. With unpaved shoulders, drop-offs can also occur as a result of rutting caused by vehicle tires or erosion.
The Problem with Drop-Offs
Even when a vehicle encounters a drop-off on a straight portion of road, while transitioning into an adjacent travel lane, it is not unusual for the driver to be startled; this unexpected condition is sometimes significant enough to cause a loss of control. For drivers who encounter drop-offs that take them out of the travel lane, their primary concern is usually to return to the travel lane as quickly as possible. In attempting to do so, it is common for the driver to encounter resistance from the vehicle’s tires “scrubbing” against the side of the drop-off. This resistance/scrubbing can cause an unexpectant driver to become more forceful in trying to get their vehicle back onto the travel portion of the road. This aggressive steering, back towards the travel lane, often results in drivers overcompensating, which can cause a loss of control and potentially a crash.
Allowable Drop-Offs Vary by State
Most states in the US dictate allowable temporary edge drop-off heights during roadway resurfacing projects. Some states do not allow temporary drop-offs above 2 inches while other states don’t permit any temporary drop-offs at all. The highway engineers at Robson Forensic can help you to understand the specific drop-off allowance that applies in your case.
A Safer Alternative
The asphalt paving technique called the Safety Edge is gaining momentum across the country as State and local transportation departments strive to protect motorists from preventable crashes. Recent research has shown that almost all drivers and vehicles can recover if the edge is tapered to 30 degrees from horizontal. This durable taper, the Safety Edge, is easy to include in the paving process, provides a safer roadway edge, and a stronger interface between the pavement and the graded material. The additional cost of the asphalt edge is minimal when included as part of resurfacing projects.
There are numerous ways to minimize or eliminate edge drop-offs. Often the solution is simple and inexpensive. The federal safety edge is one solution among others.
If your case involves a vehicle crash related to a pavement/shoulder edge drop-off, the highway engineers at Robson Forensic may be able to assist.
The municipal and highway engineers at Robson Forensic are frequently retained to determine if the design, construction, or maintenance operations of roadway systems contributed to the cause of motor vehicle collisions, pedestrian strikes, or other injuries.