Roadway Departure Crashes Expert Article

In this article, Civil/Highway Engineer, Thomas Lyden provides an overview of roadside Clear Zones as they relate to roadway departure crashes. His discussion includes the hazard of obstacles in the clear zone, clear zones in rural and urban environments, and designing and/or maintaining a clear zone to reduce the probability and severity of roadway departure crashes.

Clear Zones & Roadway Departure Crashes

Roadway departure crashes involve a vehicle that leaves the roadway, encroaches on the shoulder and roadside area, and strikes a natural (e.g., trees) or man-made obstacle (e.g., culvert, sign, utility pole, fence).

There are a number of factors that may contribute to a vehicle leaving the roadway which may include:

• Design factors (curves, narrow lanes)
• Environmental factors (weather, animals or debris in the road)
• Driver factors (inattentiveness, crash avoidance)
• Vehicle issues (tire blow outs)

By creating and maintaining proper Clear Zones, roadway designers and owners can increase the likelihood that a roadway departure will result in a safe recovery by the operator or mitigate the severity of a crash that may occur.

Clear Zones

The American Association of State Highway and Transportation Officials (AASHTO) defines a clear zone as:¹

“The unobstructed, traversable area provided beyond the edge of the through traveled way for the recovery of errant vehicles.”

The clear zone generally includes the shoulder of a highway, bike lanes, and auxiliary lanes. The basic premise is that a vehicle that strays from the roadway might be able to recover and return to the roadway or come to a safe stop if the operator does not encounter an obstacle in the clear zone.

Rural versus Urban Environments

Although clear zones are often thought of as an issue only affecting interstate and other high-speed highways in rural environments, urban conditions introduce a variety of obstacles in greater number and closer proximity to the road, such as light poles, fire hydrants, and sidewalk furniture such as benches and bus stop shelters.

While operating speeds in urban areas may be lower, the density of objects is typically higher, thus these fixed objects along urban roadways can present a significant hazard.

Calculating Clear Zones

AASHTO and state DOT design manuals recommend the width of the clear zone be evaluated based on several factors including traffic speeds, volumes, and roadway and roadside geometry, including horizontal curves.

Roadway engineers evaluate these, and other appropriate factors based on the unique conditions at the site, to determine an appropriate clear zone width. The width of the clear zone may differ across states and municipalities therefore it’s important that the highway engineers and maintenance staff recognize these differences and apply the appropriate standards.

Obstacles in the Clear Zone

Obstacles in the clear zone are a recognized hazard in the industry. Roadway engineers should follow the safety hierarchy approach when dealing with such obstacles. Consideration should first be given to removing or redesigning the obstacle so that it can be avoided or safely traversed. Relocating the obstacle further from the traveled way lessens the likelihood that the obstacle is struck by an errant vehicle.

The severity of an impact can be reduced by using an approved breakaway device, which is frequently done with roadside sign supports. A vehicle may be redirected prior to striking the obstacle with a longitudinal barrier (e.g., guardrail, concrete barrier) that shields the obstacle.

Lastly, if the previous alternatives are not appropriate, or reasonable, the obstacle should be marked with warning devices.

Roadway engineers, along with other agency staff, should routinely inspect roadside areas and clear zones for improperly placed obstacles such as non-breakaway mailboxes, fences, large rocks/boulders or advertising signs and take the appropriate steps to:

• Remove the obstacle
• Redesign the obstacle
• Relocate the obstacle
• Reduce the impact severity with the obstacle
• Redirect vehicles away from the obstacle
• Warn of the obstacle

Highway Engineering Expert Investigations

The highway experts at Robson Forensic examine crash sites, use field measurements, scrutinize crash scene photos, and review infrastructure design documents to determine if highway factors or road conditions contributed to the cause of motor vehicle crashes.

For more information, submit an inquiry or call us at 800.813.6736.

Source

1. AASHTO, “Roadside Design Guide”, 4th Edition, 2011, Pg. 3-1, 3-2.

Featured Expert

Thomas Lyden, P.E., PMP

Civil Engineer & Highway Engineering Expert

Thomas Lyden is a civil engineer with an impressive background covering the entire lifecycle of highway infrastructure. Through his 40+ years of professional experience, Thomas has conducted traffic… read more.