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Hello and welcome to the American Traffic Safety Services Association’s Work Zone Safety podcast series. This podcast is based on work supported by the Federal Highway Administration under the 2011 Work Zone Safety Grant. This podcast will focus on raising practitioners’ awareness of how to remove, shield, or otherwise mitigate hazards within the clear zone.
Today’s podcast is derived from the ATSSA “Positive Protection Guidelines Training Module” and the FHWA Work Zone Safety Grant-sponsored “Traffic Control Design Specialist” training course. Another important resource on this topic is the Work Zone Safety Consortium guidance document entitled Use of Work Zone Clear Zones, Buffer Spaces, and Positive Protection Deflection Distances. Information on these resources, and many other products developed under the FHWA Work Zone Safety Grant, is available at workzonesafety.org.
This podcast will run for about 10 minutes.
Before we begin, here’s a brief note on terminology. You may already have some knowledge about general roadside design and clear zones; however, today’s topic is specific to work zone clear zones. Different agencies use different terms for the work zone clear zone, including the safety zone, drop-off protection area, worker protection area, or recovery area. For the purposes of this podcast we will use the phrase “work zone clear zone.”
Ah, excuse me? Sorry to interrupt, but I’ve heard these different terms and they seem to refer to different things. What exactly is the work zone clear zone?
Okay, simply put, the work zone clear zone is a roadside area that is either an unobstructed, recoverable area that allows a driver to regain control of a vehicle that has left the roadway or it is a traversable area that allows a driver to stop safely. There is an important distinction between the terms “recoverable” versus “traversable.” A recoverable slope is flatter, which means a motorist should be able to steer a vehicle back to the intended path. A traversable slope means that the roadside area has a slope of between 1 to 3 and 1 to 4, and a motorist that leaves the traveled way has a clear runout area.
Due to the dynamic nature of the work zone environment, recoverable designs are achieved first by not allowing unprotected hazards created by construction activities (such as drop-offs) within the work zone clear zone area and second by shielding unavoidable hazards like utility poles with positive protection devices. Traversable designs are achieved by maintaining the minimum allowable side slope of 1 to 3 in a hazard-free location that usually requires a significant roadside width for high-speed roadways.
How big should the work zone clear zone be?
The work zone clear zone distance is usually established by a predetermined process that includes risk factors like traffic speed and traffic volume, which varies by State. For example, Florida uses an approach like the one recommended in the AASHTO Roadside Design Guide, which bases work zone clear zone distances on speed for both travel lanes and auxiliary lanes. Illinois’ process is similar but adds average daily traffic ranges within defined speed ranges and also has work zone clear zone distances for both front slopes and back slopes. Refer to the Use of Work Zone Clear Zones, Buffer Spaces, and Positive Protection Deflection Distances at workzonesafety.org for more details on selected State specifications.
Many DOTs use a simple chart that indicates the work zone clear zone distance for a given speed on low, moderate, and high speed roadways. The distance may vary anywhere from 10 to a maximum of 30 feet. The reason the clear zone may not need to be very wide is that the majority of errant vehicles intruding into a work zone clear zone do not traverse the entire distance of the clear zone. For example, more than 90 percent of vehicles leaving the travel lanes of an Interstate travel only 2 feet from the lane before regaining control or stopping. Of the 10 percent that travel further than 2 feet, only about a quarter travel more than 30 feet out from the travel lanes. (Source: Work Zone Safety Consortium, Guidance on the Use of Work Zone Clear Zones, Buffer Spaces, and Positive Protection Deflection Distances, 2014.)
Well, that makes sense but what if there isn’t enough room for the clear zone based on my State’s design process?
Often times there may not be enough distance on the roadside to provide an adequate work zone clear zone, so you’ll need to adapt your design strategy to the actual work zone conditions. You’ll probably also need to use engineering judgment to select treatments that optimize safety in the space available.
Before we talk about treatments, let’s talk a little about what we mean by work zone hazards. Examples of hazards include:
- Fixed objects such as utility poles, bridge columns, trees or non-break away sign posts;
- Stockpiles of construction materials such as rock, pipe, or removed roadway materials;
- Shoulder drop-offs, steep slopes, or other vertical conflicts that affect vehicle recovery;
- Parked equipment and devices such as work trucks, paving machines, portable changeable message signs, arrow boards, and workers’ vehicles; and
- Any unyielding or fixed objects greater than 4 inches in height.
So what are the options for treating hazards in the work zone clear zone?
Generally you have three choices: relocation, mitigation, or shielding.
Relocation refers to hazards that can be moved. This would include stockpiled materials, parked equipment and vehicles, post-mounted signs, and other fixtures. Keep in mind that the work zone clear zone concept is especially important when no work activity is taking place because during construction it is often necessary to have equipment, construction vehicles, and materials in the roadside area. For longer duration activities and for times when these hazards must remain in the clear zone because they are needed for work zone activities or because the location allows no other place to put them, it will be important to consider protection like portable concrete or steel barriers. Many States have specific requirements regarding the placement of materials and construction vehicles both during and after work hours, so be sure to consult your State and local standards.
When relocation is not possible, mitigation, or doing things to make a hazard less dangerous, can be a good compromise between maintaining the work zone clear zone and shielding hazards. However, there are limitations that must be considered, including constructability, time duration, and roadway width and length. For example, constructing a 4 to 1 wedge of compacted surfacing material to smooth drop-offs may be possible with shoulder delineation and proper signing of the drop-off condition. The longer the duration of work, the more practical this approach becomes. Additional mitigation examples include:
- Temporary grading of new excavation and embankments to create a recoverable slope of at least 3 to 1.
- Planned work operations that make use of existing barriers, wide shoulders, and an accessable roadside to eliminate the potential hazards of stockpiled material or equipment.
- Work zone strategies that shift traffic away from existing or new hazards. For example, temporary lanes shifted onto an existing shoulder or temporary widening may create enough distance to provide for the clear zone.
Lastly, shielding, or applying protective devices to make a hazard less dangerous, is very common in work zones since the work zone itself often creates hazards. In addition, existing hazards in work zones must be shielded when relocation and mitigation are not possible. It is common that the active work area be shielded with temporary barriers, such as portable concrete barriers, because of the need to separate live traffic from construction activities. Drop-offs, equipment, and fixed obstacles such as lamp posts or large trees are just a few hazards that can have a dangerous impact on vehicles that accidentally intrude into the work area. Here are some options for shielding the work area:
- Install temporary barriers. As mentioned, concrete or steel barriers are available and mobile versions may be applicable for fast-moving work operations.
- Deploy truck-mounted attenuators, or TMAs. TMAs provide a crash-worthy barrier that protects workers from intrusions into the work zone.
- Use mobile barrier vehicles. Similar to TMAs but larger, mobile barriers are mounted on a tractor-trailer. They provide a visual and physical barrier between an active work zone and live traffic and protect workers from vehicle intrusions.
This wraps up our discussion of hazard identification and work zone clear zones. This podcast has been a presentation of the Federal Highway Administration’s Work Zone Safety Grant Program. For more information on grant products, please visit the Federal Highway Administration’s Grant page on the work zone safety information clearinghouse at workzonesafety.org. For a list of resources that can provide additional guidance on this topic, please see the transcript of this podcast.
Resources for Further Review
FHWA Work Zone Safety Website — https://safety.fhwa.dot.gov/wz/
AASHTO Roadside Design Guide — https://bookstore.transportation.org/collection_detail.aspx?ID=105
ATSSA “Traffic Control Design Specialist Training Course” — http://www.atssa.com/TrainingCertification/CourseInformation/TrafficControlDesignSpecialistTCDS.aspx
MUTCD Part 6 — https://mutcd.fhwa.dot.gov/htm/2009/part6/part6_toc.htm
FHWA Rule on Work Zones Implementation Guidance — https://ops.fhwa.dot.gov/wz/resources/final_rule.htm
Work Zone Safety Consortium, Use of Work Zone Clear Zones, Buffer Spaces, and Positive Protection Deflection Distances. National Work Zone Safety Clearinghouse — https://www.workzonesafety.org/files/documents/training/fhwa_wz_grant/RSP_Clear_Zones_Guidance.pdf
Don’t forget your own State and local agency resources!