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The hazards of pavement edge dropoff.

THE roadway shoulder has been recognized as desirable ever since engineers began paving our roads. Unfortunately, roadways are often resurfaced without restoring the adjacent shoulders to bring them up to the resurfaced road level. This condition can lead to vehicle tires dropping off of the pavement edge, and, subsequently, to an accident. The following discussion is confined to two-lane rural roadways within the United States, although some recommendations are also valid for multi-lane facilities.

Some governmental agencies choose to resurface roadways with their own work forces, while others choose to turn over the entire maintenance procedure to a private maintenance contractor. Still others choose to contract a portion of the roadway maintenance work.

If the latter course is taken, a contract may be written requiring the contractor to resurface the roadway only from edge of pavement to edge of pavement, excluding the restoration of roadway shoulders. The shoulder restoration is then to be accomplished by the responsible governmental agency. In many cases, a timely shoulder restoration is performed and a safe driving surface put in place.

However, in other cases, when the resurfacing is accomplished and the contractor has fulfilled his contractual requirements, he is compensated and then released from the project. Meanwhile, the shoulder has not been raised to the same elevation as the edge of pavement and will not be until the government crews can get to the project.

During the time the roadway has a pavement edge dropoff, safety may be severely compromised, and depending upon the height of the dropoff, a vehicle hazard may be evident. One result of a pavement edge dropoff is that when a driver attempts to bring a tire back onto the roadway, the dropoff acts as a barrier against the tire, preventing it from easily reentering the roadway. This barrier is directly influenced both by the effective edge height of the pavement edge dropoff and whether or not the tire is scrubbing against the pavement edge. If the right front tire is scrubbing, the driver turns the tire sharply toward the roadway, and is applying more steering force to the tire than would be necessary if the tire was not scrubbing. When this condition occurs and the tire mounts the dropoff, the driver has usually oversteered, which then results in the driver losing control of the vehicle. As a consequence of this loss of control, any of the following may occur:

* The vehicle may have a head-on collision with or side-swipe an oncoming vehicle in the opposing lane of travel.

* The vehicle may strike the ditch or other physical obstacle on the opposite side of the roadway.

* The vehicle may go into a skid on the roadway and perhaps overturn.

* The driver may over-compensate to the right and strike the ditch or other physical obstacle on the right side of the roadway.

* The condition may be corrected and no accident occurs.

Both tire and vehicle size are substantial factors in the influence a particular pavement edge dropoff has on a vehicle's ability to reenter the roadway. As the tire size (height) increases, pavement edge dropoff has less influence on the ability of the tire to mount the pavement edge. Likewise, a larger vehicle uses momentum and sheer weight to overcome the resistance force of the pavement edge.

Situations that can lead to a pavement edge dropoff are not purely due to the installation of the new layers of pavement added to the existing roadway surface. In many cases, the existing roadway shoulder may have been worn away by traffic before installation of the new pavement. As these areas of wear vary along a roadway edge, so does the amount of dropoff.

Thus, existing dropoffs may vary from negligible up to several inches. Areas of greater dropoff usually occur where drivers have to make changes in direction of their vehicles, such as in curves.

When a typical 2 1/2-in. overlay of new asphalt is applied, a nominal dropoff can easily exceed 4 in. In these situations, the driver may be encountering a difficult section of roadway, which may result in the tires dropping from the roadway surface as many had before the resurfacing. Now the dropoff condition has reached a magnitude that can seriously alter the ability of the tire to reenter the roadway.

Since research and common sense show that a sloped edge is easier for a tire to climb, the use of an asphaltic fillet or angled edge would help alleviate the problems associated with pavement edge dropoffs. Research has shown that the use of a beveled edge, with a bevel angle of 45 [degrees], greatly reduced the control problems attributable to edge drops. Installation of this asphaltic fillet can be easily accomplished with today's modern paving equipment.

By simply attaching a device known as a "moulding shoe" to the paver, the asphaltic fillet can be formed along the pavement edge as the overlays are placed on the roadway surface. The moulding shoe not only forms the shape of the asphalt fillet, but also reduces the amount of hand work required to finish the pavement edge.

Compaction of the asphalt fillet can then be accomplished by the use of an edge compacting device attached to the compaction rollers. This device consists of a hydraulically powered wheel that rolls alongside the compactor's drum while simultaneously pinching the edge of the mat towards the drum and providing lateral resistance.

Different types of moulding shoes are in use today in the paving industry. Many are fabricated by individual contractors to fit their own paving screeds and may be adjusted to provide various angles or side slopes at the edge of a paving pass.

In general, the lay-down costs associated with using the moulding shoe are insignificant. Also, additional material quantities and costs are minimal for the added fillet wedge. For example, when resurfacing a typical 24-ft wide, two-lane rural roadway with two 12-ft lanes, the following layers of asphalt are used: a 1 1/2-in. leveling course and 1-in. surface course, for a total of a 2 1/2-in. overlay. The corresponding asphalt volume required per mile of overlay is 978 cu. yd. The volume of asphalt required to place a 45 [degrees] fillet along each side of the same roadway is 8 1/2 cu. yd. This fillet wedge volume equates to less than one percent of the roadway asphalt overlay requirement.

Many rural roadways have dropoffs of 1 to 2 in. or more before resurfacing is undertaken. In such cases overlays with 45 [degrees] angle fillets will require somewhat more asphaltic concrete, but not an excessive amount. For example, where the dropoff could be as much as 4 1/2 in. (2 1/2 in. of new asphalt and 2 in. of existing dropoff), the asphalt volume required to place a fillet along both sides of a 1-mile length of roadway would be 27.5 cu. yd, or 2.8 percent of the roadway asphalt overlay requirement.

Benefits of using an asphaltic fillet are profitable for both the long-term life of the roadway and the short-term repaving project. These benefits include:

* Increased safety during construction, due to the ease of bringing a tire back onto the roadway surface after it has dropped to a shoulder that has not been raised to the new roadway surface elevation.

* Increased future safety for drivers trying to reenter the roadway once the shoulder material is again worn away over time by tires on the roadway shoulder and by erosion.

* Added protection from roadway drainage for the roadway base and sub-base materials.

The National Research Council's Transportation Research Board has determined: "that pavement edge drop hazards are greater than previously believed" and that "In addition, pavement edge drops are a common source of tort claims against highway agencies." Therefore, the authors recommend that the most effective way of solving the problems associated with pavement edge dropoffs is to simply eliminate the issuance of contracts where shoulder work is excluded or not included in the resurfacing contract. Governmental agencies must be educated and made aware of the risk and liability associated with these types of contracts.

If, for whatever reason, the governmental agency still believes there is a need to issue "no shoulder work" contracts, as a minimum one or a combination of the following recommendations should be implemented into resurfacing contracts to provide safer roadways and to help minimize the exposure to liability:

* Require that shoulder materials be pulled up to the new surface as a non-pay item.

* Require that appropriate signing remain installed along the roadway to inform the motoring public of the existence of a low shoulder condition.

* Require that a 45 [degrees] angle asphalt fillet be installed as a part of the roadway resurfacing, along the edge of the roadway.
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Publication:Public Works
Date:Dec 1, 1994
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