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ASK THE EXPERT

Frequently Asked Questions

Worker measuring safety edge


Speaker Notes

This presentation, Ask the Expert, provides an overview of Frequently Asked Questions, along with the answers. The presentation can be used in conferences, open house presentations, pre-meetings to project demonstrations, and/or training sessions.

The opening cover photograph is from a demonstration project completed in Wapello, Iowa in 2009. The Iowa Department of Transportation (DOT) has recently developed a standard specification and adopted a policy for implementing the Safety Edge, a standard practice in all Hot Mix Asphalt (HMA) overlay applications and on concrete pavements.

Jerry Roche (FHWA Safety Engineer, Iowa Division) in the background was a key champion for the implementation in Iowa and in the national effort.

Chris Wagner (FHWA Pavement Engineer, Resource Center) is in the foreground of the photograph, demonstrating the Safety Edge. Chris, along with Frank Julian (FHWA Safety Engineer, Resource Center), who is not shown in the photograph, were key players in the initial development and implementation of the Safety Edge in Georgia, 2003. Georgia DOT has since adopted the Safety Edge as a standard practice.


Q. What Is the Safety Edge?

Images of the Safety Edge


Speaker Notes

The Safety Edge is a simple but highly effective solution to reduce crashes on rural two-lane highways. Shaping the edge of the pavement to 30 degrees minimizes the problem of vertical drop-off. This angle provides a safer roadway edge that allows drivers to re-enter the paved road safely. The Safety Edge also improves pavement density, which makes the edge durable.


Q. How does the Safety Edge compare with conventional asphalt paving processes?

Speaker Notes

Compared with the conventional hot-mix asphalt paving process, the Safety Edge improves pavement density as well as driver safety. The shoe that makes the Safety Edge can be installed on new or existing asphalt resurfacing equipment. The attachment acts as a screed extension and extrudes the asphalt, forming a compacted pavement edge in the desired 30-degree shape. Using the Safety Edge adds little to construction costs. (Resource: Safety Edge Toolkit, includes Fact Sheet comparing the Safety Edge to conventional paving.)


Q. Has the Safety Edge design been used for concrete pavement?

Speaker Notes

Yes.


Iowa PCCP Safety Edge

PCC Paving


Speaker Notes

The Safety Edge has been constructed with PCC pavements.

Conventional concrete forming attachments can be added to the paver to create the Safety Edge profile.

We do not expect many applications of the Safety Edge with concrete pavements, but it has be done.

The cost factors are different since the material is all additional. However the typical concrete edge is the shape A, which is the most severe edge shape and the safety edge would provide a significant safe improvement.


Q. My State wants to do a Safety Edge project with an Open House so interested DOT personnel, local agencies and contractors can observe an installation. Can we get a loaner shoe and technical assistance in planning and presenting information?

Speaker Notes

The FHWA has provided many loaner shoes to our Division Offices, and the Safety Edge Implementation Team has worked jointly with our Divisions to assist State and local agencies interested in hosting a Safety Edge Open House. Please contact us early in the process so we may guide you through some of the issues in coordinating an activity like this, and we can develop a target time frame and schedule your Open House.


Q. How should a State DOT or local agency select the best site for a Demonstration Project?

Speaker Notes

While the Safety Edge is applicable to a very wide range of paving projects, for your first project it is important to ensure a visual success. Therefore, it is a good idea to choose a project where the Safety Edge (which may be deeper than the lift thickness) will be at least a few inches high.

If the people working closest with the project want it to fail, they can probably make it so. Therefore, it is a good idea to work with a district or county that is open to change and interested in making improvements. The same goes for the contractor – if you have the opportunity to select the project after it has been awarded. A contractor who frequently files claims is probably not the best to work with.

Having the project within a couple hours drive of your office and the State Department of Transportation’s (DOT) Headquarters provides more opportunity for specialists to be on-site to problem solve, if necessary. It typically also means more people will have the opportunity to see it during construction, as well as monitor it afterwards.

In order to clearly show the safety benefits of installing the Safety Edge, it is preferable to select a project where at least most of the length has recoverable slopes. Similarly, the highest safety benefits are found on rural, high-speed roads with narrow or no shoulders. So while there is still a benefit to applying the Safety Edge at these locations, it isn’t recommended as a demonstration project.

When doing an open-house, it is fairly important to find a project with several miles of paving. The key here is to have a project where the contractor is likely to be paving with the Safety Edge at least several days. The open house can then be scheduled later in the project, so that if there are any project delays, you won’t need to postpone the open-house. This also allows the paving crew some time to work out any paving issues that may arise.

We typically do a slide presentation before bringing the participants to the project site, so it is helpful to have this within a short distance of the project – particularly if the need to ride-share to the project site is necessary.

We have found that you can draw a larger audience to your open-house if there are other innovations on the project as well. And, of course, the easier it is for many DOT and local highway agency personnel to get to, the more likely you are to get someone who is willing to try another Safety Edge project.


Selecting a Demonstration Project

For Your First Project

  • Existing edge drop-offs OR minimum 2” overlay
  • Progressive district/county
  • Cooperative contractor
  • Near central office
  • Recoverable area beyond pavement edge
  • Rural, higher-speed road
  • Narrow or no shoulders

For an Open House

Important:

  • Longer project (mileage)
  • Located near a conference room

Optional:

  • Warm-mix asphalt or other innovation
  • Easy travel location

Q. What is the minimum lift thickness that merits the use of the Safety Edge?


Slide 10

Lift thickness does not correlate with edge depth.

The lift of asphalt is 1.5 inches as can be seen at the centerline. Across the road it shows about a 4 inch depth because the shoulder was lower after clipping the shoulder.

Speaker Notes

Lift thickness does not correlate with edge depth.


Q. If a project has paved shoulders, is the Safety Edge needed, and if we have rumble strips do we need it?


Line Graph: GM Proving Ground Accidents (211 Cases)


Speaker Notes

General Motors monitored run-off-the-track incidents on a test track over about a 10-year period.

The area adjacent to the track was relatively flat, so these values would only apply to flat side slopes.

Analysis of these incidents revealed that 80% of the test drivers stopped or recovered within 30 feet of the track.

They also found that 40% recovered within 10 ft.

This also means that 60% go beyond the edge of a 10 ft paved shoulder.

You can see the graph for the shoulders less than 10 ft.


An edge rut is different from a drop-off. Usually this type of rut is from agricultural equipment running on the shoulder.

View of an edge rut


Speaker Notes

An edge rut is different from a drop-off.

Usually this type of rut is from agricultural equipment running on the shoulder.

The rumble strip alerts the driver that he is leaving the road.

The rumble strip will not stop the vehicle but it will give the driver advanced perception of leaving the road and the driver reaction starts earlier in the event.

The vehicle is traveling at an angle across the shoulder.

The angle of the vehicle departure, vehicle speed, and shoulder width will determine the time (fraction of seconds) it will take to reach the pavement edge and interact with graded material.

It is conceivable the safety edge will improve RS performance since both address low angle departure.


A roadway without paved shoulders and a edge rut.


Speaker Notes

Of course a roadway without paved shoulders and a edge rut are a much worse risk.

The road is more likely to have a rut and drivers are more likely to get to the rut.

Do you think drop-offs don’t occur on aggregate shoulders?


Aggregate shoulders can have settlement and over time result in a drop-off or an edge rut.

Aggregate shoulder


Speaker Notes

Aggregate shoulders can have settlement and over time result in a drop-off or an edge rut.

Notice the raveling at the edge of the pavement.

This shoulder could have benefited from a safety edge design.


edge drop-off



Aggregate shoulders in a tangent section perform better than sod shoulders since the shoulders resist water and wind erosion better. However, routine maintenance is still required, which often can become a compromise in tough budgets.

Aggregate shoulder


Speaker Notes

Aggregate shoulders in a tangent section perform better than sod shoulders since the shoulders resist water and wind erosion better.

However, the aggregate shoulders still require routine maintenance that is often a compromise in tough budgets.

It is often thought that a thin lift won’t produce tire scrubbing, but when the adjacent graded material is below the pavement lift and result in deep drop-off (and particularly a rut) then scrubbing could occur.


tire scrubbing on the edge


Speaker Notes

See tire scrubbing on the edge of this concrete pavement.


Q. What if the graded material adjacent to the pavement slopes is greater than the Safety Edge?


side slope is steeper than the safety edge angle


Speaker Notes

If you have a project that the side slope is steeper than the safety edge angle, then the side slope is not recoverable.

The safety edge can only help safety if the vehicle is recovering.

However if you have a paved shoulder like this project, you may elect to apply the safety edge to make the pavement edge more durable by consolidating the pavement edge.

This type of condition is more vulnerable to raveling, because of the lack of shoulder support.

However, if the project only has intermittent segments like this roadway in the photograph, you may still improve a good part of the project and it should be considered.


Q. What are other alternates to the Safety Edge?


paving vehicle edge design


Simulation Cases

  • Edge designs
    • Geometric criterion18 edge configurations including both limited dimensions and wide wedges

  • Vehicles selected
    • 2 passenger cars, 1 pick-up truck, 1 tractor semi-trailer (loaded and unloaded configurations)

  • Driving conditions
    • 2 friction conditions to simulate dry and wet roadway

  • Driver’s reaction
    • 3 levels of steering and braking reaction times

Safety Design Criterion D

Geometric criterion: D = H/a³ Geometric criterion

with H the edge drop off height (in)

and a the edge slope angle (rad)


Dcrit

Safe and failed recovery maneuvers



Conclusion

  Dcrit (in)
Mirage 16.5
Camry 10.4
Pickup Truck 24.8
Tractor semi-trailer 3.5
  • For passenger vehicles, wedge is safely designed under all kinds of driving conditions if: D ≤ 10.4 in

  • For all types of vehicles (including heavy commercial vehicles): D ≤ 3.5 in


Q. Will the wedge shape increase erosion or cause aggregate to slide away?


Before Photos

Before safety edge photo with close-up



After Photos–Six Months After Paving

pictures show a roadway resurfaced with Hot Mix Asphalt (HMA) and incorporating the Safety Edge


Speaker Notes

The pictures show a roadway resurfaced with Hot Mix Asphalt (HMA) and incorporating the Safety Edge. The roadway width and traffic volume/mix did not change so the same vehicle off-tracking has continued. However, having the Safety Edge in place provides a safer situation when the eroding/rutting continues. The largest drop-off observed was approx. 1.75 inches, but the Safety Edge goes down over 5 inches.


Q. How much width does the wedge add?


Slide 31

Angle and degree formula


Speaker Notes

The shoes available today by the manufacturers will accommodate a little less than 5 inches.

This was based on the 80’s researchers that drop-offs more than 5 inches would allow the small vehicles undercarriage to snag on the pavement edge.

While we still have small cars in the fleet, the majority of vehicles in the fleet have more ground clearance.


Q. What happens when you come to a driveway?

http://fhwa.na3.acrobat.com/sedriveways/

Speaker Notes

The video shows the smooth transition to the paved apron.

However, a rough transition is provided coming out of the driveway and requires a small amount of hand work.

This is because the end gate is pulled back through the pavement as it angles back to the through-lane edge.

It does not require stopping the paving train.

The screed operator will have to adjust the shoe with the adjusting screw.

For slight grade variations, the spring will compress and allow the shoe to ride up—but it is likely at a driveway the adjusting screw will need to adjusted.


Q. What happens when a future resurfacing project comes, will the pavement be much wider?



Existing pavement or base



Q. How do you pave under the guardrail with the Safety Edge shoe attached?

Road with guardrail



Paving under the guardrail



Paving under the guardrail



Q. This looks to be simple. Can we make our own device, and how difficult is it to install the manufacturer’s shoe?

http://fhwa.na3.acrobat.com/seshoeinstall/


Q. When you are doing a project with double lifts, do you need the Safety Edge for both lifts?

http://fhwa.na3.acrobat.com/seddlift/


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Page last modified on May 18, 2012.
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