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Q: What is the Safety EdgeSM?
A: The Safety EdgeSM is a procedure to shape the edge of the pavement at approximately 30 degrees from the pavement cross slope during the paving process. The Safety EdgeSM eliminates tire scrubbing, a phenomenon that contributes to the loss of vehicular control. It has been successfully constructed on both asphalt and concrete pavements.
Q: How does the Safety EdgeSM compare with conventional plant mix asphalt paving processes?
A: Conventional paving results in a near vertical, unconsolidated edge. The Safety EdgeSM hardware extrudes the hot mix asphalt into a more durable and safer 30 degree edge where the paved surface meets the unpaved shoulder.
Q: How does the Safety EdgeSM compare with conventional Portland Cement Concrete (PCC) paving processes?
A: Conventional PCC paving results in a vertical edge. Constructing the 30-degree Safety EdgeSM on PCC pavement is similar to constructing integral curb shapes. Preliminary results indicate this shape reduces stresses at the edge of the slab.
Q: Why would I change my current practice to include the Safety EdgeSM?
A: The Safety EdgeSM improves the short- and long-term safety of the roadway and improves the durability of the pavement. Studies have shown that severe crashes often occur when drivers lose control attempting to traverse a near vertical pavement edge to reenter the paved roadway. Research has shown virtually all drivers can recover, even at high speeds, if the edge shape is 30 degrees rather than vertical. Consolidation at the edge of pavement using the Safety EdgeSM devices has shown excellent stability and has potential to reduce pavement edge raveling.
Q: What type of highways or roads should have the Safety EdgeSM?
A: Any pavement without curbs will realize safety and pavement benefits with the Safety EdgeSM. There are potentially greater benefits on roads with narrower pavement, such as those with no paved shoulder.
Q: Could the Safety EdgeSM be added to an existing pavement as a project independent of paving/resurfacing?
A: Not at this time. Installing the Safety EdgeSM during the paving process is critical to achieving a durable pavement edge. There is some limited experience in applying the Safety EdgeSM during shoulder widening projects.
Q: Are there any suggested criteria for selecting a demonstration project?
A: There is some benefit in selecting a project where the need for the Safety EdgeSM is obvious. Therefore a rural road with narrow or no shoulders with existing drop-offs and otherwise recoverable slopes is ideal. Since existing drop-offs are unlikely to be known, selecting a project with at least 2-inches of new pavement is an alternative. In order to overcome any issues that might arise, there is also benefit in selecting a project near your office or in a district or county with good problem solvers.
Q: How should an agency select the best project for an open house to demonstrate the Safety EdgeSM?
A: Since paving schedules are prone to change due to many factors, a key consideration is a project with many miles of paving so that you can plan an appropriate date for the open house where observers can see actual paving. It is also useful to select a project located within easy travel distance of your guests, where a conference room is available nearby. It is also critical to make arrangements ahead of time for parking and observing the paving operation.
Q: Can we get a loaner Safety EdgeSM device and technical assistance in planning and presenting information?
A: Loaner devices have been provided to all interested FHWA Division Offices. In addition, technical assistance is available for open houses, demonstration projects, and evaluation as resources allow. Please contact Christopher.Wagner@dot.gov or Frank.Julian@dot.gov for more information.
Q: Does implementing the Safety EdgeSM have any legal implications?
A: Many states and contractors have been sued over edge drop-offs from construction, weather, or traffic. The Safety EdgeSM helps reduce these types of crashes and indicates a good faith effort to provide due care.
Project Development Questions
Q: We use thin lift overlays for resurfacing. Do we need the Safety EdgeSM?
A: The Safety EdgeSM is recommended even for thin overlays to achieve long-term safety benefits as well as improved pavement durability. Often existing edge drop-offs, cross slope, and/or clipping of the shoulders during construction will result in the outside edge of the overlay being thicker than the lift over the existing pavement surface.
Q: How much will the addition of the Safety EdgeSM cost per mile?
A: It will be almost negligible for single lift plant mix asphalt projects. It does depend somewhat on the specific design and construction parameters. When measured, it has been calculated to be less than 1% additional asphaltic material. Use of the Safety EdgeSM on PCC projects will likely see a measurable increase in materials that is easily calculated. Some contractors have seen savings with the Safety EdgeSM due to more efficient operations in pulling shoulders up and reduced damage repairs to edge of pavement by haul trucks.
Q: What is the maximum height and width of the Safety EdgeSM?
A: This may vary by device, but most currently have a maximum height of about 5 inches, resulting in a maximum width of about 8 ½ inches. Regular maintenance should prevent drop-offs of greater than 5 inches occurring except in rare circumstances.
Q: If I use the Safety EdgeSM do I still need shoulder backing?
A: Yes. Use of the Safety EdgeSM does not take the place of shoulder backing material. The ultimate surface should be a horizontal plane comprised of the paved lanes with shoulder backing material. The best practice is to pull the adjacent material flush with height of the pavement surface. The purpose of the Safety EdgeSM is to provide short-term mitigation to edge drop-offs and a longer window for practical maintenance scheduling. It also provides a short-term benefit during construction. For those jurisdictions that do not typically bring shoulders back up following an overlay, use of the Safety EdgeSM would significantly lower crash risk. Each state will be different as to when the shoulder backing must be placed.
Q: Does the Safety EdgeSM have to be placed on gravel?
A: No, the Safety EdgeSM has been constructed over shoulders of crushed stone, Reclaimed Asphalt Pavement (RAP), and in-situ soil and performed well. Like any pavement, better performance may be expected with better base to provide support.
Q: Is the Safety EdgeSM beneficial for a project with aggregate shoulders?
A: Yes. Aggregate shoulders also settle and erode over time resulting in a drop-off or an edge rut can develop from vehicle encroachments.
Q: Is there a benefit to installing the Safety EdgeSM on roads with paved shoulders?
A: Yes. Paved shoulders provide an opportunity for many vehicles to recover before leaving the pavement, but even with six-foot or wider shoulders, some errant vehicles are going to leave the pavement. If they encounter a drop-off at that point, the chance of safely returning to the roadway is going to be reduced unless a Safety EdgeSM is used. In addition, it is expected the pavement edge will be more durable.
Q: What if a roadway only has limited clear zone available?
A: Obviously, the more clear zone available, the better chance drivers have to recover and attempt to return to the pavement, but even roads with limited clear zone can benefit from the Safety EdgeSM.
Q: What is the significance of the 30 degree wedge? Why not 20 degrees or so like embankment slopes?
A: The original research showed a 45 degree edge was adequate, but field testing during device development found the 30degree wedge more constructible. Further research indicates this also provides an added measure of safety. This design has been found to be the most practical in terms of safety, constructability, and pavement edge durability.
Q: Will pavement get wider with each resurfacing project that includes the Safety EdgeSM?
A: No. Assuming you keep a consistent top width, the wedge will typically be placed on the new shoulder material. So if the drop-off height is similar to the last resurfacing project, the wedge will be approximately the same height and width.
Q: Are there any environmental issues with placing the Safety EdgeSM?
A: There are states that have environmental laws regarding the disturbance of vegetation on the shoulder and/or drainage of runoff water from the pavement. These situations need to be addressed on a case-by-case basis.
Q: If a project has rumble strips do we need the Safety EdgeSM?
A: Rumble strips may alert a driver that they are leaving their lane, but some will still leave the pavement. The Safety EdgeSM provides a better opportunity for the driver to safely recover.
Q: What if the slope of the graded material adjacent to the pavement is steeper than the Safety EdgeSM slope?
A: There will be some cases that the Safety EdgeSM will not help vehicles recover, due to steep slopes directly adjacent to the pavement. In these instances an agency may benefit from the pavement durability aspects of the Safety EdgeSM in those areas of the project.
Technical Questions (Pavements/Maintenance/Safety)
Q: What are the effects of the Safety EdgeSM on the pavement quality and structure?
A: Results from a test section constructed in 2003 show a reduced deterioration of the pavement edge in comparison to control sections built without the Safety EdgeSM. In general, quality of the pavement is mostly dependent on factors other than the edge – in place density, mixture components and overall construction quality. Over many installations the Safety EdgeSM has been proven to have no adverse effects on the pavement.
Q: Do I need to change my mix design or layer thickness to use the Safety EdgeSM?
A: The Safety EdgeSM has been constructed with most typical Superpave and Marshall mix designs used for resurfacing. Nominal aggregate sizes of 19.0mm (3/4 inch), 12.5mm (1/2 inch) and 9.5 mm (3/8 inch) have been used successfully in the construction of the Safety EdgeSM to date.
Q: Is the Safety EdgeSM durable? Does the toe break or wedge come apart over a period of time?
A: Initial Safety EdgeSM demonstration projects were constructed in 2003 and in these sections the Safety EdgeSM is still performing well. There has been no breakage even in sections where logging trucks repeatedly traversed the edge. Indications are that the Safety EdgeSM provides an additional level of consolidation on the edge, decreasing edge raveling and increasing pavement life.
Q: Will corridors with the Safety EdgeSM require more maintenance or less?
A: In-service evaluations have reported that the Safety EdgeSM is no better or worse at holding the shoulder material flush with the pavement. Therefore, agencies should plan for the same cost of maintenance operations. However, in locations where the shoulder material does settle or move, the Safety EdgeSM shape provides a safer situation until such time that maintenance operations can be scheduled and mobilized.
Q: On multiple lift construction, is the Safety EdgeSM constructed on each lift or only the final lift?
A: The Safety EdgeSM has been successfully constructed with each alternative. When placing on successive lifts, allow for the additional top width on the bottom lift and tolerance in lining up the top wedge over the bottom. If the Safety EdgeSM is placed on the final lift only, the Safety EdgeSM device will only shape the top 5 inches or so. This is considered acceptable as drop-offs of greater than 5 inches should be rare with regular maintenance.
Q: Can I construct the Safety EdgeSM with Warm Mix Asphalt (WMA)?
A: Yes. It has been constructed successfully on a number of WMA projects.
Q: Can open graded friction course (OGFC) or rubberized hot mix asphalt-gap graded (RHMA-G) be used with the Safety EdgeSM?
A: OGFC is not recommended as this thin of a lift with a one size stone most likely will not form the Safety EdgeSM through the device. No issues have been documented in using the RHMA-G with the Safety EdgeSM.
Q: What type of prep work is needed for the shoulder?
A: It is recommended that the standard agency shoulder prep work be performed. If removal of the soil/vegetation at the pavement edge (clipping the shoulder) is standard practice, it may be necessary to clip an additional few inches of material to accommodate the paving equipment as it is a few inches wider when constructing the Safety EdgeSM than conventional paving. The Safety EdgeSM is constructed of plant mix asphalt, so paving on heavy vegetation may result in an uncompacted wedge of asphalt. If the project scope includes widening the pavement, it is recommended that additional widening be included in the project to accommodate the Safety EdgeSM.
Q: What subgrade compaction should be specified where the Safety EdgeSM will sit?
A: There is no compaction requirement under the Safety EdgeSM.
Q: How is the Safety EdgeSM compacted and what is the percent compaction?
A: The Safety EdgeSM is not compacted by the roller. The shape is extruded through the Safety EdgeSM device which provides some compaction and has been proven to produce a durable edge.
Q: Do I need to install the Safety EdgeSM if my crash data does not indicate we have a pavement edge drop-off problem?
A: Crash data rarely detects pavement edge drop-offs, however, a 2006 AAA report documented the problem is prevalent and crashes of this type are more likely to result in fatalities. Installing the Safety Edge system-wide as roads are paved is recommended.
Q: What type of crash reductions can I expect from the Safety EdgeSM?
A: Based on a recent study, the best current estimate is a 5.7% reduction in all crashes. The Safety EdgeSM is a proven safety countermeasure based on numerous field tests with a wide variety of vehicles, drivers, and speeds. According to TRB State of the Art Report 10, “...the transition from on-roadway surface to shoulder and back [using the 30 degree shape] is so smooth it defies assignment of any degree of severity, except when the elevation change from pavement to shoulder causes a noticeable tilt in the vehicle.”
Q: What is the cost of the devices that form the Safety EdgeSM?
A: Recently we have been quoted between $2,500 and $5,000 depending on which manufacturer and whether you need the edge on both the left and right side of the paver. Keep in mind, this is a onetime cost per machine – or they can use the same device and move it to another machine. The device is made out of hardened steel so it should last as long as the other pieces of equipment such as the screed.
Q: How do I obtain the commercially available device?
A: Currently, there are at least four manufacturers producing equipment that can create a quality Safety EdgeSM. Devices are available from:
Transtech Systems, Inc.
1594 State Street
Schenectady, NY 12304
Advant-Edge Paving Equipment LLC
33 Old Niskayuna Road
Loudonville, NY 12211
Carlson Paving Products
18425 50th Ave. E
Tacoma WA 98446
Troxler Electronic Laboratories, Inc.
3008 E. Cornwallis Rd. • PO Box 12057
Research Triangle Park, NC 27709
Q: How long does it take to install the Safety EdgeSM device?
A: The Safety EdgeSM devices can be installed on most asphalt pavers in less than 15 minutes. Some pavers may require shims to allow the device to sit plumb. Contractor mechanics have successfully installed these on a number of projects.
Q: Is there a minimum lift thickness where the Safety EdgeSM, is constructible?
A: The Safety EdgeSM has been used successfully in overlays as thin as 1 inch. Typically the outside edge of the overlay will be a little thicker due to drop-offs, cross slope, and/or clipping of the shoulders before the overlay is constructed.
Q: This looks to be a simple device. Can we make a generic version?
A: We do not recommend fabricating your own device. Successful generic devices have required several prototypes to achieve the smooth finished edge and necessary compactive effort for performance. Any generic device used on a project should be tested to ensure they achieve similar results as commercial devices.
Q: Can I use my Michigan Shoe (or a similar commercial device) to pave the Safety EdgeSM?
A: A few State and local agencies have tried this with little success. These devices are designed to aid in longitudinal joint density (typically the center line joint). The devices are available from the same manufacturers who make Safety EdgeSM devices, however their design and purpose are different. The defining problem with using one of these devices is in the range in movement available. Typical Safety EdgeSM devices can adjust 5 inches below the screed to accommodate the anticipated variation in shoulder grade, whereas the longitudinal joint only needs to accommodate a consistent lift thickness.
Q: Does using the Safety EdgeSM change production rate?
A: There is no lost time in production rate when using the Safety EdgeSM.
Q: Do I need to modify my paving process to install the Safety EdgeSM on asphalt?
A: Very little. The key item is to add a Safety EdgeSM device per manufacturer’s instructions to create the Safety EdgeSM. While paving, the device needs to be monitored and periodically adjusted to keep the bottom edge of the device in contact with the road shoulder surface.
Q: Does the Safety EdgeSM affect finished ride quality?
A: The initial research project monitored sections constructed with and without the Safety EdgeSM. No statistical difference between the control section and the Safety EdgeSM section smoothness was apparent. Many sections have since been constructed and there has been no indication that the Safety EdgeSM will affect finished pavement smoothness.
Q: What is the length of an average test strip to determine performance of a Safety EdgeSM device?
A: In discussions with several contractor screed operators, the basic function of the device is understood after paving a short distance (e.g. 1000 feet), but as with other experiences the screed operator will learn more as they encounter various paving conditions such as variable depth drop offs, side roads, guardrail, or steep slopes.
Q: Can a Safety EdgeSM device be placed on both sides to accommodate paving a narrow road in a single pass?
A: Yes. This has been done a couple of times with no negative feedback.
Q: When paving very narrow widths, how does the Safety EdgeSM device work with a block that narrows the screed width?
A: Several of the commercial devices require the screed be extended about 10 inches. If the narrow width is just a short section, the device can be removed in about 20 minutes so the screed extension can be pulled in. In the case of entire roadway sections that are very narrow, alternatives such as finding a paver width a narrower screed width or closing the road and paving full width in one pass could be considered, depending on contract requirements and flexibility.
Q: What happens when you come to a driveway when using a Safety EdgeSM?
A: The operator will need to raise the device to disengage it. A small amount of hand work may be needed before and after the driveway.
Q: How do you pave under the guardrail with the Safety EdgeSM device attached?
A: When paving next to the guardrail one can disengage the Safety EdgeSM device and perform paving as is typically done.