Every year in the United States more than half of the nation’s estimated 34,000 traffic
deaths involve roadway departures. When a vehicle leaves the roadway, it typically ends
up overturning or hitting a fixed object. In some cases, vehicles leave the roadway
after colliding with another vehicle, resulting in severe crashes. Crash victims can be your
friends, family members or co-workers. They can be ordinary people who may have become
momentarily distracted or forced from their lane by another driver. Whatever the cause,
measures must be taken to minimize these tragic roadway departure crashes.
One cause of roadway departure crashes, particularly on rural two-lane highways, is pavement-edge
dropoff, the uneven edge or vertical drop between the paved travel lane and the unpaved
shoulder. But there is an easy and low-cost solution
to pavement-edge dropoff. By attaching a simple device to a paving machine, you can create
a more durable and safer pavement edge. By making this simple improvement your organization
can reduce roadway departure crashes and save lives.
The problem of pavement-edge dropoff begins at the time of conventional construction.
The new pavement typically adds a vertical dropoff that should be mitigated.
Many agencies mitigate the dropoff by regrading the adjacent unpaved material. But as the
regraded shoulder settles and erodes in the weeks and months after construction, the dropoff
is exposed once again. While that’s happening, another problem
may develop. Without sufficient edge durability, the new pavement edge may start to crumble
and deteriorate. A pavement-edge dropoff can create problems
when drivers leave the pavement and drop onto an unpaved surface. The inside of the vehicle’s
right-side tires can start to rub intensely against the vertical edge of the pavement,
causing what is referred to as tire scrubbing. This can make it difficult for a driver to
safely re-enter the paved travel lane. Drivers attempting to return immediately to the paved
roadway can over-steer and lose control of their vehicles.
Studies have shown that crashes resulting from pavement-edge dropoff are more severe,
typically resulting in vehicles colliding head on with oncoming vehicles. Or they may
cross the centerline and run off the other side of the road and overturn in a ditch or
hit a fixed object. An effective and inexpensive countermeasure
to pavement-edge dropoff is to install an angled edge called the Safety Edge.
The Safety Edge is one of several technologies now being deployed as part of the Every Day
Counts initiative, which has been developed by Federal Highway Administrator Victor Mendez
to offer a better, faster and smarter way of building America’s highways. The initiative
focuses on shortening the time it takes to deliver major projects to the public and getting
promising technologies deployed into the field faster so they can enhance safety, reduce
congestion and save energy.
Studies have shown that providing a 30-degree angled edge eliminates tire scrubbing, making
the pavement edge safer to traverse for drivers and cyclists.
This angled edge can be easily installed at very low or no cost along the pavement edge
during any paving project. A special commercially available Safety Edge
shoe can be attached to a paving machine in a few minutes. As the paving material extrudes
out of the paver, the shoe forms a compacted pavement edge of the appropriate shape.
Once the Safety Edge shoe is attached and the paver is running normally, the process
proceeds the same as any other paving operation. For asphalt paving, the spring and the various
angles of the shoe extrude the material into a tight shape that provides the specified
edge. This diagram shows a typical Safety Edge installation,
with the break point located above the edge of the existing pavement. During normal construction,
the dropoff will exceed the thickness of the lift. If the project requires a second lift,
it is recommended to place the safety edge during each lift. The break point of the second
lift can be aligned over the existing pavement edge, or if the pavement width allows, the
break point can be moved in, resulting in a narrower footprint. The wedge shoes that
are currently manufactured allow for a 5-inch Safety Edge height. Most experts feel that
if the total height at the edge is more than 5 inches, a small vertical edge below the
safety edge will have negligible impacts on performance, since it would be unusual for
more than 5 inches to become exposed. Rather than the loose 45-degree angle that
comes out of the standard paver and later tends to break off, the Safety Edge shoe produces
the 30-degree wedge that’s so durable it can withstand the weight of trucks and other
heavy vehicles. To get the best results, the adjacent shoulder
material should be pulled back from the existing pavement, so that the Safety Edge sits on
undisturbed material rather than vegetation or loose gravel. When the shoulders have been
clipped back, the edge is often a little thicker than the rest of the new asphalt mat. This
is perfectly normal and acceptable. Even with the Safety Edge installed, the shoulders
should be pulled back flush with the pavement surface. As the shoulder material settles,
wears down and erodes, the Safety Edge is there as a safety net until the shoulder is
restored. This gives highway agencies more time to do shoulder maintenance or do other
tasks, since it will be less critical to replace material when the edge has only a few inches
exposed. The Safety Edge was originally developed for
asphalt paving. But the Safety Edge can also be used for concrete paving when the concrete
interfaces with graded material. In the slip-forming process used in concrete paving, different
shapes are available to make integral curbs. The Safety Edge is simply a variation of that
concept, an inverse of the commonsloping curb. As with asphalt pavement, adjacent shoulder
material should be brought flush with the concrete pavement.
One of the many benefits of the Safety Edge is that very little additional time and materials
are required for paving. Installing the Safety Edge uses less than 1% of additional asphalt,
and it requires no additional labor beyond attaching the shoe to the paver. The shoe
itself is inexpensive relative to the cost of the entire paving project and can be used
for many projects before needing replacement. For concrete paving, additional material is
required. The quantity can be easily calculated based on the pavement depth. If you are assuming
two 12-foot lanes and a 6-inch depth, you would need about 3½ % of additional material.
Research has shown that with this angled edge, tire scrubbing will not develop so that most
drivers recover safely after encountering a dropoff.
The Safety Edge is a proven technology with two primary benefits. In addition to enhancing
safety, the Safety Edge creates a more durable pavement edge that prevents edge raveling.
The Safety Edge has a proven record of success.
If you would like to learn more about the Safety Edge and how to deploy this promising
new technology in your state, go to this Internet site or contact the individuals listed.
Because of the potential benefits of the Safety Edge, your organization is encouraged to consider
adopting the Safety Edge as a low- or no-cost solution to pavement-edge dropoff. Why not
make this simple improvement to reduce roadway departure crashes and save lives in your state?