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Safety Edge Technical Overview

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THE SAFETY EDGE THE PURPOSE, NEED, AND PRACTICAL SOLUTION

Purpose and Need

  • Crash Types and Problem Locations
  • Risk Factors in Edge Drop-off Crashes

A Practical Solution

  • Construction of the Safety Edge
  • Durability
  • Benefits and Costs

Conclusion

Speaker Notes

Safety Edge: Technical Overview

The purpose of this technical overview is to introduce the purpose and need for the Safety Edge; the practical solution the Safety Edge provides; and the features and benefits. This presentation also is an opportunity to answer questions and discuss the advantages of the Safety Edge.

Three messages are key to communicating the benefits of the Safety Edge. The Safety Edge—

  • Saves lives.
  • Is low cost.
  • Improves durability.

This Safety Edge Technical Overview contains the following:

  • Purpose and Need
  • A Practical Solution
  • Conclusion

This is your elevator speech to all your partners.  The Safety Edge-

  • Reduces crashes and saves lives by mitigating pavement edge drop-off
  • Is a low cost, systematic improvement applied during paving

Improves durability by reducing edge raveling Communicate that the Safety Edge is a simple but extremely effective solution that can help save lives by allowing drivers who drift off highways to return to the road safely.  The FHWA's goal is to accelerate the use of the Safety Edge technology, working with States to develop specifications and adopt this pavement edge treatment as a standard practice on all new and resurfacing pavement projects.

Approach to Reducing Roadway Departure Crashes

  • Low-Cost Solutions
  • Highly-Effective Countermeasures
  • Systematic Application
Roadside view of a Safety Edge

Speaker Notes

67% of all fatal roadway departure crashes occur on rural highways, most of them on 2-lane roads.

This is the type of roadway where the Safety Edge can have the most benefit.

The Safety Edge is a solution to a problem that can occur on almost any roadway, which can be incorporated into our systems as we pave and repave the roadways at very little cost. As you’ll see in the following presentation, there are both safety and pavement longevity benefits to be gained with this low-cost improvement.

The Safety Edge can and should be applied to other roadway types (divided and pavement with paved shoulders, and urban roads without curbs), but these type of pavements are the most critical in terms of potential fatality reduction.

Locations at High-Risk for Drop-Offs

  • Horizontal Curves
  • Near Roadside Mailboxes
  • Turnarounds/Unpaved Pull-Outs
  • Shaded Areas
  • Eroded Areas
  • Asphalt Pavement Overlay

Speaker Notes

When surveying a candidate project for drop-off locations, you most likely will notice the condition is prominent in the following locations. The following slides take a closer look at these areas.

Horizontal Curves

Close-up view of measuring a Safety Edge


Speaker Notes

Common occurrence in horizontal curves due to tire wear. Also super-elevation accounts for drainage and erosion on the inside of the curve.

Mail Boxes

A mail box on the side of road


Mail delivery vehicles often leave the paved surface causing edge rutting. This condition can be found at nearly 100% of mailboxes w/o paved turnouts. Also, the mailbox may be a roadside hazard in and of itself.

This is an example of an extreme location.

Turnarounds/Unpaved Pull-Outs

Images related to turnarounds/Unpaved Pull-Outs


Shaded Areas

Sunlight = Vegetation


Notice how grass starts exactly where the sunlight hits. Also, this in in a slight horizontal curve. Cut trees back would remove obstacles and allow light for vegetation.

Florida places sod adjacent to the paved surface to encourage growth.

Eroded Areas

Image of eroded areas


Asphalt Overlay

2”Asphalt Overlay + Existing 5” Drop-off = Extreme Unsafe Condition


It is important to note that the addition of the Hot Mix Asphalt (HMA) overlay increases the drop-off height. Inclusion of the Safety Edge Detail can show that the agency is providing “Due Care” to mitigate existing and created drop-off locations.

With Safety Edge

Diagram: With Safety Edge


Risk Factors

What are the factors associated with pavement edge drop-off crashes?

  • Speed
  • Driver Experience
  • Vehicle/Tires
  • Drop-off Height
  • Shape Of Pavement Edge

Speaker Notes

Speed, Driver Experience, Vehicle cannot be controlled—

Drop-off height is what agencies typically try to control, but it is intermittent and varies by location

Shape of the pavement edge is the only thing we can control over the life of the pavement!

Risk Factors: shape of pavement edge


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


Line Graph: Relative Degree of Safety/Longitudinal Edge  Elevation Change (inches)


This graph was developed as a result of the findings of research at Texas Transportation Institute and published in Transportation Research Board (TRB) first State of the Art Report, The Influence of Roadway Surface Discontinuities on Safety January 1, 1984.

In this study they only rated shape A, B, and C.

The Safety Edge: The Practical Solution

Worker measuring a safety edge


Construction

  • Similar to Conventional Paving (No Effect on Production)
    • Clip Shoulders
    • Construct Overlay
    • Pull Shoulders Flush
Conventional paving work

Speaker Notes

The process of asphalt resurfacing with the Safety Edge varies little from the conventional method other than the addition of the shoe to the paver.

As always, it is preferred to clip the existing shoulder to eliminate any debris or soft soil and weeds.

The overlay is then placed and the shoulders pulled flush with the top of the pavement.

Paving proceeds at the same rate as conventional paving.

The device requires only a minimal amount of monitoring by the screed operator to ensure the shoe is tight to the existing shoulder.

The Safety Edge has been applied to a number of different SuperPave mixes and even installed on a warm-mix asphalt.

The Hardware

Trans Tech Shoulder Wedge Maker™ Advanti-Edge™
www.transtechsys.com
www.troxlerlabs.com
www.advantedgepaving.com

Speaker Notes

Trans Tech Hardware connects to the screed not to the end gate.

This is a major difference between the commercial hardware and the Georgia design.

The Trans Tech device uses a spring to produce the downward force and to allow for the vertical jog.

Rolling Process

Rolling process


Speaker Notes

In the demonstration project the roller followed the roller pattern in the approved mix design.

The roller pattern was typical to the mix used and incorporated a vibrator roller that did not hang over the breaking point of the pavement edge.

Recent projects have shown that some mixes are more tender and may require some considerations and it varies due to the mix , temperature, AC content, etc.

On occasion the roller hanging over the edge by 6 inches has worked well, where in other projects the roller has stayed away from the edge until it is cooler.

The concern is the roller will push the face more vertical and exceed the 35 degrees tolerance.


Safety Edge without Shoe Safety Edge with Shoe
http://fhwa.na3.acrobat.com/sewoshoe/ http://fhwa.na3.acrobat.com/sewoshoe/

Speaker Notes

Note particularly the loose material coming out of the paver not using the safety edge shoe.

This material will be lost as the pavement ages.

This will lead to raveling and an inconsistent edge.

Note also the straight line at the pavement edge, the tight and smooth look of the pavement by the paver using the shoe.

Iowa PCCP Safety Edge

PCCP Safety Edge


Speaker Notes

The safety edge has also been constructed with PCC pavements.

Conventional concrete forming attachments can be added to the paver to create the SE 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.

Finished Surface

flush shoulder


Speaker Notes

The Safety Edge is not an alternative to a flush shoulder.

It is a temporary safety measure during construction and a permanent feature for where edge rutting occurs in the future.

Routine maintenance of shoulders should be conducted to maintain a flush shoulder.

Angle Measurement

Close up of the edge showing the angle measurement


Speaker Notes

This is a close up of the edge showing the angle measurement.

The Safety Edge should create a 30o angle (up to 35 degrees for construction tolerance) as measured at the toe of the slope (the vehicle re-entry point).

Angle Measurement

method to measure the angle method to measure the angle
Mobile, AL Installation Columbus, MS Installation

Speaker Notes

Two methods to measure the angle. Note that the angle measurements are precise to approximately +/- 3-5 degrees.

Finished Surface

finished surface with piece removed


Finished Surface

piece of the finished surface


Speaker Notes

Notice the shape of the HMA has conformed to shape of the graded shoulder and shape of the previous pavement.

Lift thickness does not correlate with edge depth.

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

Speaker Notes

Lift thickness does not correlate with edge depth.

Drop-Off with the Safety Edge

Shows drop-Off with the Safety Edge


Speaker Notes

Recent research shows that drop-offs seem to recur at the same rate with the Safety Edge as without.

This location was a drop-off prior to resurfacing and was pulled flush after resurfacing.

This picture shows that drop-offs will recur at the same location since the Safety Edge is designed to mitigate the drop-off effects and not stop the erosion that caused the drop-off.

Note the good stand of sod beyond this drop-off, which is typical through this project.

Note also the wedge is large at this location, because there was a previous drop-off.

If you were to go to the sod area and pull back the soil you would note the wedge is not as large since the shoulder drop-off was not as deep so the wedge was not as large.

Increased Edge Durability?

Edge Durability without Safety Edge Edge Durability with Safety Edge

Speaker Notes

Notice the crack on the edge of the pavement and the loose material sloughing off when the conventional paving process is used.

Contrast this to the finished appearance of the safety edge.

The lateral confinement produces greater compaction at the pavement edge.

This provides additional support to the pavement and reduces edge raveling.

Comparison of Edges

Comparison of paving with and without the Safety Edge (Maine Safety Edge Installation)


Speaker Notes

This photo compares edges immediately behind the paver (before the hot-mix asphalt was rolled). Note the difference in slope on each side of the straight edge. Visually the material is better compacted with the Safety Edge than the material without the Safety Edge.

Photo note: Maine demo project, August 2010

Edge Durability

Maine Control Section w/o Safety Edge


Speaker Notes

Looking more closely at the section where the Safety Edge shoe was not used on the project (about 100 feet from the location shown on the previous photo), note the crack in the edge highlighted by the pen sticking in it. The asphalt beyond the crack will break off and be wasted.

Edge Compaction

Condition After 6 Years of Service
Without Safety Edge With safety Edge

Speaker Notes

Six years after this Safety Edge installation, the shape is still in ‘Like New’ condition.

The control section without the safety edge has raveled to a near vertical edge (Shape B in previous graph).

Notice the cracking that is beginning to develop near the edge on the section w/o the safety edge.

These are the locations where there were edge drop-offs noted in the preconstruction evaluation.

Even though these shoulders were pulled back flush with the pavement the drop-offs re-emerged after just one year. Can you guess why??

These Locations were both in full shade during most of the day and grass did not re-establish.

Durability

Asphalt truck rolling over a Safety Edge
http://fhwa.na3.acrobat.com/setruck/


Speaker Notes

On a project in Iowa, we noticed that the loaded asphalt trucks were routinely rolling over the Safety Edge that had been paved the day before.

There was no physical deterioration where the truck crossed back onto the pavement.

Tracy’s Law: “If you lose the edge, you lose the road.” – Tracy Cumby, TxDOT Project Director (Photos Courtesy  of Dr. William Lawson, Texas Tech  University)
http://fhwa.na3.acrobat.com/setruck/


Speaker Notes

On a project in Iowa, we noticed that the loaded asphalt trucks were routinely rolling over the Safety Edge that had been paved the day before.

There was no physical deterioration where the truck crossed back onto the pavement.

Texas Maintenance Assessment Program (TxMAP)

Bar Graph: TxMAP 2002 Non-Interstate Assessment, District Overall Summary


Speaker Notes

(Reference Dr. William Lawson/Texas Tech TRB Presentation)
TxMAP is the acronym for the Texas Maintenance Assessment Program, and this is TxDOT’s statewide system whereby they evaluate a statistical sample of all their roads for the pavement condition, traffic operations, and roadside. TxDOT is now in their 4th year of collecting these data.

For the three available years, they all look about like this. The bars represent the 25 Districts. In TxMAP, there are some 21 items that are rated, and the two most significant for our purposes are “OVERALL” and “Pavement Edges”. What I want to point out is...
These TxMAP data show a remarkable amount of correlation between edge condition and overall performance at the top, and more particularly, the bottom, of the rankings. GOOD NEWS/ BAD NEWS...

Let us assume that an important goal for TxDOT personnel would be to achieve a District overall ranking at or near the top of the overall rankings for the State. The TxMAP data in this Table show that, in order for a District to rank as one of the top 3 districts in the State overall for non-interstate assessment, it was necessary to rank in the top 9 in the State on edge maintenance. This correlation suggests that good edge maintenance strategy is necessary for good roads.

But the correlation is even stronger at the bottom of the list. With the exception of one anomaly, for all three years data have been collected, those districts that ranked in the bottom 3 of the State overall fell within the bottom 4 on edges. Stated another way, if a District ranked at the bottom of the list on edges, it was the bottom of the list overall...

What this means is that good edge maintenance strategy is not only important in achieving good roads, but without good edge maintenance, a District cannot achieve good roads. Stated simply, the TxMAP findings support Tracy’s Law, “If you lose the edge, you lose the road.”

It is on this basis we contend that good pavement edge maintenance strategy is perhaps the key element of a successful highway maintenance program.

When Maintenance Section personnel think of their job in terms of Tracy’s Law, their focus and priorities become clear. This is very profound, and ranks as one of the central findings of this research.

Benefits of the Safety Edge

  • Temporary safety benefit during construction
  • Increase production–shoulder work after overlay complete
  • Providing “Due Care”
  • Aid vehicle re-entry
  • Increased Pavement Edge Durability
  • Reduced Crashes Over Life of the Pavement

Speaker Notes

Temporary—for before the shoulders are pulled back flush and permanent for when edge rutting does occur in the future.

This may also allow for a construction expedient.

The contractor may be able to postpone the regrading of the shoulders until after the completion of the overlay.

Other Safety Measures

  • Build 2-foot shoulders
  • Install rumble strips/stripe
  • Periodically rebuild/maintain shoulders
  • Use aggregate, or RAP shoulders

Speaker Notes

This slide is included to recognize that there are other techniques to reduce the frequency or severity of ROR accidents. The SE can be used in coordination with these efforts or when these details are not feasible.

Costs of the Safety Edge

  • Hardware

    • Approximately $3000 per shoe
    • Reusable
  • Material

    • Minor additional asphalt (depends on shoulder condition)
  • Paving Process

    • No change in paving speed
    • No additional operation
    • Minimal monitoring
  • Surface Details

    • No change in smoothness/ride qualit

Every Safety Edge Counts

The Safety Edge provides benefits to all stakeholders: owners, contractors and the driving public.

The Safety Edge saves lives and improves pavement edge durability.

The Safety Edge costs less than 1% of pavement resurfacing budgets.

YOU can help reduce pavement edge drop-off crashes!

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Page last modified on August 12, 2013.
Federal Highway Administration | 1200 New Jersey Avenue, SE | Washington, DC 20590 | 202-366-4000