[Taken from MAC summary:

Part IV -Subject of Discussion: Pavement Traction

Shoulder drop-off: pavement is overlaid for the ride quality, preserving

the pavement structure, and not allowing you to get wet –which causes an

edge.  Motor vehicles are able to get back on the road, but motorcycles

are not.  Doesn’t feel anything can be done.]

Taken from Public Roads Magazine September/October 2007 · Vol. 71 · No.

2

www.tfhrc.gov/pubrds/07sep/01.htm 

The Low-Cost Dropoff Solution

by Steve Moler

The safety edge, a relatively easy and inexpensive countermeasure to

steep pavement edges, is reducing crashes on rural two-lane highways.

[Picture: James Leben, an assistant district maintenance engineer with

the Georgia Department of Transportation, measures a tapered safety edge

applied in a 2004 demonstration project along a 21-kilometer (13-mile)

section of State Route 88 near Augusta, GA. The safety edge is one means

to help prevent crashes caused by pavementedge dropoffs.]

Four teenage boys from a high school in Clayton County, GA, were driving

to school on a rural two-lane highway on a March morning in 2003 when

something went terribly wrong.

About a quarter-mile from campus, the car's right tires slipped off the

pavement and dropped onto the sandy shoulder. While attempting to return

to the pavement, the 16-year-old driver overcorrected and lost control.

The compact sedan crossed the centerline and slammed head-on into a

school bus coming in the opposite direction. The driver and one

passenger were pronounced dead at the scene; another passenger died

later at the hospital. The fourth teenager was seriously injured but

eventually recovered. The driver of the school bus, which was carrying

no passengers, suffered only minor injuries.

One cause of the crash, according to the police report, involved a

condition known as pavement-edge dropoff (PEDO), the uneven edge or

vertical dropoff between the paved travel lane and the unpaved shoulder.

Highway safety experts consider a dropoff of 12.7 centimeters (5 inches)

or more to be unsafe, especially if the edge is at a 90-degree angle to

the shoulder surface. A dropoff of 5.1 centimeters (2 inches) or more is

considered a potential driving hazard. The dropoff along the stretch of

highway where the teens' car slipped off the pavement ranged from about

5.1 to 10.2 centimeters (2 to 4 inches), according to two safety

engineers from the Federal Highway Administration (FHWA) who visited the

site the day after the crash.

When a vehicle slips off the pavement and onto an unpaved shoulder, the

steep edge can make it difficult for a driver to reenter the paved

travel lane safely. Studies show that when a driver encounters a steep

pavement edge, he or she attempts to return immediately to the paved

travel lane but in doing so tends to oversteer, causing intense rubbing,

or "scrubbing," of vehicle tires against the pavement edge, which

initially prevents the vehicle from climbing back onto the pavement.

This oversteering can cause loss of control at the moment when the right

rear tire climbs back onto the pavement, causing the vehicle to fishtail

or go into a broadside skid.

[Picture: FHWA Safety Engineer Frank Julian uses his shoe for scale to

demonstrate the extent of the pavement-edge dropoff along the section of

roadway where three teenagers were killed in a crash.]

"This is likely what happened in the Lovejoy High School crash," says

FHWA Safety Engineer Frank Julian, who visited and took photographs of

the crash site and learned that there was evidence of scrubbing on the

inside edge of the vehicle's right tires. According to the police

report, skid marks were found coming back onto the roadway, leading the

inspectors to believe that "overcorrection played a role in the

[crash]."

Although relatively rare compared with other crash types, PEDO-related

crashes tend to be more severe, say the authors of a recent study

sponsored by the AAA Foundation for Traffic Safety. In fact, these

crashes are more likely than others on similar roadways to result in

serious injuries and are two to three times more likely to be fatal,

primarily because the vehicle often leaves the roadway, rolls over, hits

a roadside object, or is involved in a head-on collision. According to

FHWA, an estimated 11,000 people suffer injuries and roughly 160 die

annually in crashes related to unsafe pavement edges.

"Pavement-edge dropoff has been around for a long time, and it will

continue to be a serious problem unless we do something different," says

former FHWA Chief Highway Safety Engineer Rudy Umbs, who now works in

the agency's Resource Center. "We need to keep asking ourselves, What

are we going to do differently tomorrow and next week to eliminate

pavement-edge dropoffs and reduce the potential for lane departure

crashes like run-off-the-road and head-on crashes? The safety edge can

make that difference."

One solution is to install a 30- to 35-degree tapered asphalt wedge or

fillet, known as a safety edge, along each side of the roadway during

resurfacing projects. The safety edge not only provides an angled and

compacted transition that eliminates the abrupt dropoff, but it also

provides for a stronger and more stable pavement edge, which makes it

easier for drivers to maneuver their vehicles safely back onto the

roadway. By offering a tapered, rather than vertical, transition between

the paved surface and the unpaved shoulder, the safety edge is a

low-cost means of improving safety on paved two-lane highways.

[Picture: At the time of the crash, the highway, shown here, was a

typical two lane rural road with unpaved shoulders. Recently this

section of road was upgraded to four lanes due to commercial and

residential development in the area.]

Factors in PEDO Crashes

Numerous studies over the past three decades have analyzed the various

combinations of conditions and circumstances that lead to PEDO-related

crashes, such as the one involving the Georgia teens. Virtually all of

the studies found that whether a driver regains control or crashes after

slipping off the pavement edge depends on a variety of circumstances,

including vehicle speed, steer angle, the vehicle's departure and return

angle, vehicle size, dropoff severity, driver skills, roadside

obstacles, and whether another vehicle is coming in the opposite

direction. In the Georgia incident, as in many PEDO-related crashes,

several dangerous circumstances converged to create the conditions

conducive to a crash.

[Picture: Red paint, as well as FHWA Safety Engineer Frank Julian's

foot, marks the spot where the car reentered the paved travel lane and

went into a broadside skid, colliding head on with a school bus coming

in the opposite direction.]

First, according to police findings and eyewitness accounts, the driver

was likely exceeding — perhaps far exceeding — the 72

kilometer-per-hour, km/h (45 mile-per-hour, mi/h), speed limit. The car

was a small sedan, which studies, including one conducted by researchers

at the University of Michigan Transportation Research Institute, show

have more difficulty recovering from pavement-edge dropoffs. The dropoff

at the crash site was within the range that experts consider unsafe, and

the driver was inexperienced, having had his driver's license for just

13 days. And finally, a vehicle — indeed, a fairly large one — was

coming in the opposite direction. To complicate the situation, none of

the teenagers was wearing a safety belt.

Magnitude of the Problem

A variety of conditions in the roadway environment can contribute to

PEDO, including pavement-edge breaking, erosion, wear of the unpaved

shoulders, or inadequate maintenance. A Transportation Research Board

(TRB) report, Construction of a Safe Pavement Edge: Minimizing the

Effects of Shoulder Dropoff, indicates that edge dropoff most commonly

is encountered around mailboxes, on the insides of curves, on steep

grades, at turnarounds, and along shaded areas. A combination of

shoulder erosion and edge rutting caused by harsh weather and vehicles

repeatedly leaving the paved travel lane typically is found at these

locations.

Another circumstance that can aggravate PEDO is failure to bring the

shoulder flush with the pavement following a resurfacing project.

Problems develop when the pavement edge begins to crumble quickly from

the lack of compaction, creating a vertical drop. Edge rutting and soil

erosion from repeated vehicle impacts and the weather soon follow.

Just how serious is the PEDO problem overall? In 2006 the AAA Foundation

for Traffic Safety sponsored a comprehensive study that attempted to

answer that question. Conducted by the Iowa State University Center for

Transportation Research and Education (CTRE) and the Midwest Research

Institute, the study, Safety Impacts of Pavement Edge Drop-offs, found

that most States routinely sample edge dropoffs for maintenance

purposes. But that information is either not available or only indicates

that a sampled section exceeds a certain threshold — in a sense, passes

or fails, without giving a more descriptive assessment.

To gain a clearer understanding of the magnitude of the PEDO problem,

the AAA Foundation studied randomly selected sections of paved rural

two-lane highways with unpaved shoulders in two Midwestern States. The

study found that 12 percent of dropoffs sampled in one State were 5.1

centimeters (2 inches) or more, 1 percent were 7.6 centimeters (3

inches) or more, and less than 1 percent were 10.2 centimeters (4

inches) or more. In the other, the situation was slightly worse. Almost

19 percent of the dropoffs sampled there measured 5.1 centimeters (2

inches) or more, 3 percent were 7.6 centimeters (3 inches) or more, 1

percent were 10.2 centimeters (4 inches) or more, and less than 1

percent were 12.7 centimeters (5 inches) or more.

The researchers found that in most cases, States are aware of edge

dropoffs as an issue and, in many cases, have aggressive maintenance

policies in place. However, edge dropoff persists due to harsh weather

and traffic conditions. State and local agencies also have significant

highway mileage to maintain and might not always be aware of all the

locations where dropoffs have formed, according to Shauna Hallmark, a

transportation engineer and professor in the Department of Civil,

Construction, and Environmental Engineering at Iowa State's CTRE and the

lead researcher on the AAA Foundation's study.

Another question transportation researchers are trying to answer is how

often do PEDO-related crashes occur? Various studies in recent years

have examined this question. One study, known as the Southeast United

States Fatal Crash Study and headed by Karen Dixon, a civil engineering

associate professor at the Georgia Institute of Technology, evaluated

150 randomly selected fatal crashes on rural two-lane State and nonstate

highways in Georgia in 1997. Dixon and her colleagues estimated that in

21 of the 69 nonState-system fatal crashes in Georgia, or about 30

percent, edge rutting or PEDO was a causal factor.

The aforementioned study by the AAA Foundation also evaluated the number

of crashes where characteristics indicated that edge dropoff might have

had an impact. The researchers found that 17.7 percent of crashes on

rural two-lane roadways in one State and 24.7 percent in another were

probably or possibly related to edge dropoffs. They also found

indications that PEDO crashes are run-off-the-road crashes, which in

general are more likely to be severe than other crash types.

Evolution of the Safety Edge

In numerous studies over the years, researchers have sought to

understand the conditions that lead to PEDO crashes. In 1982, the Texas

Transportation Institute conducted one of the first studies on the

advantages of using an angled wedge along the pavement edge to minimize

dropoff severity. The theory behind a tapered pavement edge was that it

could help drivers make a smoother, more controlled reentry onto the

paved travel lane than if there was a more abrupt or vertical edge.

[Picture: A researcher is using a ruler and level to measure

pavement-edge dropoff where rutting and erosion have occurred on a rural

road in Iowa.]

In a 1986 study, researchers Don L. Ivey, a civil engineering professor

emeritus at Texas A&M University, and Dean L. Sicking, a civil

engineering professor at the University of Nebraska-Lincoln, analyzed

the steer angle needed to remount dropoffs with different heights and

edge shapes at 80 km/h (50 mi/h). A 10.2-centimeter (4-inch) vertical

edge generally caused loss of control, but as the edge shape became

flatter, fewer impacts were felt. The researchers then evaluated 5.1-,

10.2-, and 15.2-centimeter (2-, 4-, and 6-inch) dropoffs with a

45-degree wedge and found that drivers could recover within the

3.7-meter (12-foot) travel lane even with as much as a 15.2-centimeter

(6-inch) dropoff.

That same year, researchers at the University of Michigan Transportation

Research Institute compared vertical and 45-degree wedge dropoffs with

hard and soft shoulders, various passenger vehicle sizes, and front- and

rear-wheel-drive vehicles using nonprofessional drivers. The results

showed that none of the nonprofessional drivers could negotiate a

vertical dropoff of 11.4 centimeters (4.5 inches) or higher at any

speed. Dropoffs near 7.6 centimeters (3 inches) could be negotiated at

speeds of 48 km/h (30 mi/h) in large passenger cars, but smaller cars

needed lower speeds to recover.

But with a 45-degree edge, drivers always were able to recover within

their own lane when traveling at speeds up to 89 km/h (55 mi/h). The

researchers also evaluated soft shoulders using a professional driver

and concluded that dropoff height, not shoulder material, was the

determining factor in being able to recovery safely.

A 2005 study by the Center for Intelligent Systems Research used

computer modeling to analyze vehicle recovery for various vehicle types,

dropoff heights, and wedge angles. Pierre Delaigue, a research scientist

with the center, presented the results from the study, Safety of

Excessive Pavement Wedge Due to Overlays, at the 2005 TRB meeting in

Washington, DC.

[Picture: The Georgia wedge, shown here, is bolted onto the screed end

gate.]

The study concluded that flatter wedges were always safer than steeper

ones, regardless of dropoff height. Tractor semitrailers were most

sensitive to PEDO, while pickup trucks were least sensitive. Passenger

cars recovered from dropoffs of up to 12.7 centimeters (5 inches),

provided a pavement wedge of 45 degrees or flatter was present. However,

a 10.2-centimeter (4-inch) dropoff with a 45-degree wedge was too severe

for the tractor semitrailer. But a 30-degree wedge allowed all vehicle

types to recover safely.

Creating the Taper

Although the idea of using a tapered wedge had been around for years,

determining a method to lay the wedge along a road shoulder during a

resurfacing project posed an ongoing challenge.

In early 2003, Resource Center Safety Engineer Frank Julian and Pavement

and Materials Engineer Chris Wagner began developing some basic concepts

for how to create a tapered edge along the roadway shoulder. First, they

built on Wagner's experience at the National Center for Asphalt

Technology, where he conducted research in the late 1990s on creating

tapered wedges at the longitudinal joints of asphalt pavement. Julian

and Wagner next conceived of developing a device that could be attached

to the end of an asphalt paver screed. The device needed to be able to

create a smooth and compacted wedge along the edge of the roadway.

[Picture: As shown here, the safety edge is measured 30 to 35 degrees up

from the horizontal at the toe of the wedge surface.]

>From this conceptual stage, Julian and Wagner developed a partnership

with the Georgia Department of Transportation (GDOT) to design and plan

a demonstration project to study the constructability of what they

termed the safety edge on a resurfacing project. GDOT began the

demonstration project in 2004 along a 21-kilometer (13-mile) section of

State Route 88 just south of the town of Augusta. GDOT's maintenance

department developed its own in-house device known as the Georgia wedge.

Conceived by GDOT Maintenance Project Manager Lynn Bean, the wedge is

essentially a modified strike-off bolted onto the screed end gate. The

shoe of the end gate rides on the pavement shoulder and moves freely

vertically, allowing it to adjust to height changes. A rounded leading

edge produces the smooth appearance.

The safety edge was successfully installed with little impact on

production and a project cost increase of less than 1 percent. After 1

year, the Georgia demonstration project found no visible signs of

deterioration and reported no expectations for any long-term

degeneration along the safety edge sections.

The sections of roadway paved without the safety edge during the

demonstration project had degraded to a near-vertical edge during that

same time, with cracking developing near the edge. The Georgia study

concluded that the safety edge showed "promise as a low-cost solution to

mitigate pavement shoulder dropoff . . . The implementation of the

safety edge design would be most applicable to asphalt resurfacing

projects on two-lane undivided roadways with limited paved shoulders."

According to highway safety experts at FHWA, the safety edge is not

intended to be an alternative to a flush shoulder, but rather can serve

as a safety feature used in conjunction with current shoulder

specifications. The recommended pavement wedge, measured 30 to 35

degrees from the horizontal, helps prevent drivers from overcorrecting

if they drift onto the shoulder, thereby decreasing the likelihood of

the vehicle crossing into opposing traffic or leaving the roadway.

"The safety edge is an ideal solution since the impact of a vehicle

encountering a vertical difference between the edge of the roadway and

shoulder can be lessened when dropoffs form and agencies aren't

immediately aware and able to address the problem," says CTRE's

Hallmark.

[Picture: Without the safety edge, this section of highway in a 2004

Georgia demonstration project developed a near-vertical drop just 1 year

after a pavement overlay.]

For these reasons and others, the safety edge is now a standard feature

of resurfacing projects in Georgia.

Other Demonstration Projects

While the Georgia demonstration project got underway, FHWA asked

TransTech Systems, Inc., to develop a device to manufacture the safety

edge commercially. The company adapted its Notch Wedge Joint MakerTM,

which creates a tapered edge at the longitudinal joint on asphalt

resurfacing projects, to produce the Shoulder Wedge Maker, which would

create the safety edge. The device attaches to the screed face instead

of the end gate and has a self-adjusting internal spring that allows it

to follow the roadside surface independently of other paver components.

The device has an angled surface that precompacts the asphalt as it

enters the device, while another fixed-angle surface forms the tapered

edge. As the asphalt is placed beneath the wedge-forming surface, it is

smoothed to a finished surface on the tapered edge.

A recent demonstration project in New York's Schenectady County using

the Shoulder Wedge Maker showed positive results. After the safety edge

was installed in 2004 along two rural roads, annual inspections revealed

that "the shoulder wedge has held up exceptionally well, with no

degradation of the edge," says Dave Clements, an associate director in

the Office of Operations Management for the New York State Department of

Transportation, who supervised the inspections. Additional analysis has

shown no cracking or breaking away of the wedge from the main rolled mat

area.

[Picture: The sections with the safety edge in the same 2004 Georgia

demonstration project showed no visible signs of deterioration.]

The Indiana Department of Transportation (INDOT) currently is involved

in an FHWA-sponsored study under the Transportation Pooled Fund Program

to evaluate the safety edge's effectiveness in helping prevent and

mitigate PEDO-related crashes. Eight companies were awarded contracts

that called for installing the safety edge in 2004 and 2005 along nine

rural, two-lane highways with minimal shoulders, using either the

Shoulder Wedge Maker or a similar device developed in-house at INDOT.

Four contractors used the existing device, while the others developed

their own versions loosely based on the Georgia wedge.

The contractors have installed the safety edge successfully on seven of

the nine projects to date, at minimal additional cost. "In fact, most of

the contractors didn't even factor the safety edge into their bids,"

says Elizabeth Pastuszka, an INDOT pavement and materials engineer who

was involved in construction of the demonstration projects. "In the two

unsuccessful projects, the problems we had were totally unrelated to the

safety edge itself."

Reducing Tort Liability

Another benefit of the safety edge is the potential to reduce tort

liability. In the 2004 FHWA report Construction of a Safe Pavement Edge:

Minimizing the Effects of Shoulder Dropoff, FHWA's Wagner and GDOT

researcher Yeonsoo Stanley Kim noted that PEDO is a common source of

tort claims against highway agencies. The authors cite court cases in

Louisiana, Minnesota, and South Carolina where monetary judgments were

awarded to motorists involved in PEDO crashes. In these cases, the

transportation agencies were found liable for creating unsafe conditions

and not warning about them.

The study by the AAA Foundation in Iowa and Missouri found that crashes

in which PEDO was the major cause resulted in major tort liability

suits. Claims filed between 2000 and 2005 in Iowa, for example, in which

"pavement/shoulder edge" or "shoulder conditions" were cited as the

major cause of the crash were the highest ranking tort liability claims

in terms of total dollar value. In fiscal years 2000-2003, these claims

accounted for 38 percent of the total dollar value of claims filed

against the Iowa Department of Transportation.

[Picture: In a typical pavement resurfacing project, the asphalt at the

edge remains soft because there is little or no compaction, as seen

here.]

>From 2000 to 2005, 23 PEDO- related tort liability claims were filed

against Iowa, the study reported. Of those, however, compensation was

awarded to the plaintiff in only two cases. Based on these findings, the

authors concluded: "The [Iowa] DOT believes [the State's] demonstrably

strong maintenance policy has contributed to [its] success in defending

against tort liability claims related to pavement-edge dropoff. When

States and other highway agencies are not able to defend themselves

successfully, edge dropoff can result in significant liability."

>From 1988 to 2003, Louisiana had 388 claims filed against it for alleged

roadway shoulder defects, including PEDO, according to former Assistant

Attorney General James R. Dawson. The State paid out an average of

$62,144 per claim, totaling more than $241 million, regardless of

whether it won or lost the case.

[Picture: A 30- to 35-degree tapered edge or fillet, as demonstrated

here by an engineer using a ruler and level, provides a compacted, and

therefore more stable, pavement edge.]

Citing these statistics at a 2004 workshop on managing pavement-edge

dropoffs in Atlanta, GA, Dawson concluded that "shoulder defects,

including dropoff problems, are a major factor in how we are able to

spend our dollars on improving highway safety."

The safety edge is indeed a promising technology that safety experts say

can help eliminate these shoulder defects and prevent tragedies like the

one near Lovejoy High School from happening again.

Guidelines and Recommendations on PEDO

Although no national standards currently exist for pavement-edge

dropoffs, several government and industry organizations provide some

guidance:

A 2006 AAA Foundation for Traffic Safety study recommends that highway

agencies require routine comprehensive sampling of PEDO on their roads,

suggesting that any dropoffs of 5.1 centimeters (2 inches) or more

should be corrected. The report also recommends that agencies adopt a

policy of providing paved shoulders with a minimum width of 0.6 meter (2

feet) wherever possible and incorporate the safety edge in all roadway

resurfacing projects to prevent severe PEDO.

The AAA Foundation further recommends that highway agencies review their

databases to assess how PEDO might have contributed to crashes and then

conduct additional research on crash occurrences and pavement-edge

hazards specifically for rural roads. In addition, the study encourages

highway agencies to train maintenance and construction staff, including

private contractors, on the potential hazards of PEDO.

In its Roadside Design Guide, the American Association of State Highway

and Transportation Officials (AASHTO) states that no vertical dropoff

greater than 5 centimeters (2 inches) should occur between adjacent

lanes, and pavement-edge dropoff greater than 7.6 centimeters (3 inches)

should not be left overnight.

The Manual on Uniform Traffic Control Devices provides recommendations

for signs used to warn motorists of unexpected conditions. For example,

if the pavement-edge dropoff is less than 7.6 centimeters (3 inches), a

"Low Shoulder" sign should be used. If the PEDO exceeds 7.6 centimeters

(3 inches), a "Shoulder Drop Off" sign is recommended.

The AASHTO book A Policy on Geometric Design of Highways and Streets

(also known as the "Green Book"), states that regular maintenance should

provide for a shoulder that is flush with the pavement surface. Unstable

shoulders generally undergo consolidation over time, and the elevation

of the shoulder tends to sink below the paved travel lane. The resulting

dropoff can adversely affect drivers when they slip onto the shoulder.

FHWA's Standard Specifications for Construction of Roads and Bridges on

Federal Highway Projects offers guidance on PEDO in work zones. The

document states that when shoulder dropoffs exceed 5 centimeters (2

inches) a "Low Shoulder" warning sign should be placed during

construction. With dropoffs greater than 10 centimeters (4 inches), a

1:3 (18-degree) fillet with "Low Shoulder" warning signs should be

provided.

Steve Moler is a public affairs specialist at FHWA's Resource Center. He

has been with FHWA since 2001, providing the agency's field offices and

partners with support in media relations, public relations, and public

involvement communications. He earned a bachelor's degree in journalism

from the University of Colorado at Boulder. He can be reached at

415-744-3103 or steve.moler@fhwa.dot.gov.

For more information about road departure issues and effective

countermeasures, please visit the FHWA Office of Safety's "Road

Departure Safety" Web site at

http://safety.fhwa.dot.gov/roadway_dept/index.htm, or contact Chris

Wagner at 404-562-3693 or christopher.wagner@fhwa.dot.gov, or Frank

Julian at 404-562-3689 or frank.julian@fhwa.dot.gov.