On August 28th the Tennessee Transportation Assistance Program (TTAP) partnered with the Federal Highway Administration (FHWA) and the Tennessee Department of Transportation (TDOT) to host a Safety Edge Open House and Demonstration event. This event, held in Nashville, allowed more than 50 participants from city and county agencies, TDOT, FHWA, engineering consultants, and contractors to learn more about how the Safety Edge can help to reduce the hazards associated with pavement edge drop-offs.
Nationally, 56 percent of fatal crashes involve roadway departures. In 2012, roadway departure contributed to 621 of Tennessee's 1,014 traffic fatalities. Pavement drop-offs, or locations where the shoulder is lower than the edge of the adjacent pavement, play a significant role in many roadway departure crashes. Researchers studying crashes in Missouri during 2002-2004 reported that pavement edges may have been a contributing factor in as much as 24 percent of rural run-off-road crashes on paved roadways with unpaved shoulders. This type of crash was twice as likely to include a fatality when compared to all rural crashes on similar roads.1
Drivers leave the paved road for many reasons. When a driver drifts off the roadway and tries to steer back onto the pavement, a vertical pavement edge can create a "tire scrubbing" condition that may result in over-steering. If drivers over-steer to return to the roadway without reducing speed, they are prone to lose control of the vehicle. This loss of control is often described as "overcorrection" in crash reports and news stories. The resulting crashes tend to be more severe than other crash types. The vehicle may veer into the adjacent lane, where it may collide with oncoming cars; overturn; or run off the opposite side of the roadway and strike a fixed object or overturn on a slope.
Inexperienced drivers are not the only victims of tire scrubbing. Smaller, lighter vehicles have a harder time climbing a steep pavement edge. At high speeds, the climb is particularly dangerous. According to inservice evaluations, a vertical or near vertical drop-off of 2.5 inches or greater has been shown to pose a significant risk.
The challenge in preventing these types of roadway departure crashes is that a drop-off is created during most paving projects. Even when the unpaved shoulder is re-graded to eliminate the drop-off, the edge often becomes exposed within a few months due to erosion or exposure to traffic. The pavement edge itself also may deteriorate.
The Safety Edge is an effective solution to reduce pavement edge-related crashes, by shaping the edge of the pavement to 30 degrees using a commercially available device (called a shoe) that can be attached to the paver. The asphalt is extruded under the shoe, resulting in a durable edge that resists edge raveling. Research has shown this 30-degree shape allows drivers to re-enter the roadway safely.
After paving with the Safety Edge, the adjacent material should be re-graded flush with the top of the pavement. This is considered the best practice, and provides the safest condition. The difference is that when the edge becomes exposed, this shape can be more safely traversed than a vertical edge. Research has shown that pavements built with the Safety Edge showed reductions of 5.7 percent of total crashes.
The event began with welcoming statements from Pam Kordenbrock, FHWA Tennessee Division Administrator, and Jessica Rich, FHWA TN Division Safety Engineer. Chris Wagner, Senior Pavement and Materials Engineer with the FHWA Resource Center, shared information on lane departure crashes and edge drop-offs, the Safety Edge concept and its effectiveness in mitigating lane departure events, and the Safety Edge construction process. Brian Hurst, Transportation Manager 2 in the TDOT Project Safety Office, discussed Tennessee's efforts to reduce the occurrence of lane departure crashes, the role of the safety edge as a countermeasure against these crashes, and Tennessee's Strategic Highway Safety Plan.
Mr. Wagner's presentation addressed a wide range of Safety Edge-related topics. Included in his presentation was a case study of a real-world roadway departure crash that resulted in the death of three Georgia high school students. This was presented as an example of the type of crash that the Safety Edge is designed to prevent. Key messages from his presentation include:
Mr. Hurst's presentation showed participants that roadway departure crashes, including those involving edge drop-offs, are a significant concern in Tennessee. In the period from 2008 to 2012, roadway departure crashes contributed to 63% of Tennessee's traffic fatalities and 42% of the state's serious injury crashes. The Safety Edge is now one of many infrastructure safety countermeasures that have been identified in the most recent update to the Tennessee Strategic Highway Safety Plan (SHSP). Mr. Hurst also shared that the Safety Edge is now a standard TDOT provision in all applicable new construction projects and on many resurfacing projects.
Following these presentations, participants traveled to the new Tennessee Traffic Incident Management (TIM) training facility at the Tennessee Highway Patrol Training Center. While on site the group was hosted by Tim Murphy, Paving Division Manager with LoJac, and Hossam Bahour, TDOT Project Supervisor. On the date of the visit, a LoJac paving crew was laying asphalt pavement on the new roadway sections that will allow traffic incident management first responders to new or refined traffic control and response coordination techniques in a controlled environment. The paver was equipped with a Carlson Safety Edge Endgate, allowing all participants to observe the equipment in action on a real-world paving project. During this time on the work site, the open house presenters and hosts were available to answer questions from participants.
To see materials (including presentations, pictures, and videos) from Tennessee's Safety Edge Open House and Demonstration, or to find links to learn more about the Safety Edge, please visit the TTAP website at http://ctr.utk.edu/ttap/index.html.
If you would like to implement the Safety Edge on your own local roadway project, FHWA has made a paving shoe available for loan through TTAP. To learn more about this equipment or to request its use, please contact Matt Cate at 1-800-252-7623 or email@example.com.
1 Hallmark et al., "Safety Impacts of Pavement Edge Drop-Offs", AAA Foundation for Highway Safety, Washington, DC, September 2006.
2 B. Powell, "NCAT Evaluates Safety Edge at Pavement Test Track", Asphalt Pavement Magazine, pp. 30-37, January/February 2014.
3 C. Wagner, "Construction of a Safe Pavement Edge", in Proc. of Transportation Research Board Annual Meeting, Washington, DC, 2005.
Knoxville, Tennessee 37996 | 865-974-1000
The flagship campus of the University of Tennessee System