A Primer on Roadway
Pavement Edge Drop Offs


John C. Glennon, D. Engr., P.E.
January 2005 (copyright)

 A pavement edge drop off (a.k.a. pavement edge drop, pavement edge dropoff, pavement edge drop-off, shoulder drop off, pavement/shoulder drop off, paved surface drop-off, etc.) is created by a difference in elevation between two surfaces of the roadway. Typical examples are:
  • The shoulder is lower than the travel lane. This condition is often created by a pavement overlay that is applied without an appropriate raising of the shoulder level. Another example is the unpaved shoulder that becomes significantly rutted or eroded away often because of the lack of proper maintenance.

     
  • Elevation differences between or within paved lanes and shoulders. These are normally temporary conditions that occur during  pavement overlay or milling operations.

     
  • The roadside area is lower than the paved shoulder.This condition is similar to the others mentioned above except that it exists further from the edge of the travel lanes.

     
When a vehicle strays from the travel lane onto the shoulder or roadside, the normal reaction of a driver is to return to the safe haven of the travel lane. The driver's urgency to make this return is usually heightened by the sudden drop when the shoulder or roadside elevation is significantly lower than the travel lane.

Pavement edge drops can cause drivers to have unexpected collisions, particularly when they are surprised at night by the sudden drop of a wheel. Most commonly, the vehicle will be affected in one of three ways: (a) move abruptly across the travel lanes and either collide with opposing vehicles or roadside hazards off the far edge of the roadway; (b) overturn on the roadway or roadside; or (c) collide with roadside hazards beyond the edge drop.

The research has generally shown that the ability of a driver to recover from a pavement edge-drop excursion is a function of edge-drop height and shape, vehicle speed and path angle, and the width of lane available for recovery. Then too, certain vehicles such as motorcycles, sub-compact automobiles, and tractor-trailer trucks have a much greater sensitivity to edge drops than do the full-size automobiles that have normally been tested.

Research1-5 indicates pavement edge drop offs of 2 inches or even lower can cause vehicular loss of control. The major hazard comes from a low-angle departure from the traveled way followed by a low-angle return which causes the tires to scrub against the edge drop. When scrubbing occurs, the wheel develops a large resistance to mounting the pavement and the driver, who is attempting to recover the travel lane, will continue to increase the steering. This resistance continues until the front-wheel steer angle is sufficient to overcome the retarding force and to create enough side force at the unobstructed front tire to lift the obstructed (scrubbing) front tire over the edge drop off. Once the obstructed tire mounts the edge, the large steer angle produces the characteristic rapid movement (slingshot effect) across the travel lane.

Although most of the research on pavement edge drop off hazard has focused on the vehicle dynamics related to a scrubbing re-entry, other accident situations are causally related to edge drop offs. Most clearly, vehicle rollovers can be caused by heavy undercarriage contact and/or other exaggerated vehicle instabilities related to edge drops of six inches or more. Then too, when drivers attempt to recover from a shoulder or roadside excursion, they will steer a circular path that requires lateral acceleration. If the radius of this path is relatively low for the vehicle speed, contact with a pavement edge drop off can trigger loss of control (much like hitting a large bump while steering a roadway curve).

That a vehicle traveling close to a 2-inch pavement edge drop off is on the knife-edge of disaster is well documented in several references. For example, A Policy on Geometric Design of Rural Highways, 19656, by the American Association of State Highway Officials (AASHO) recognizes the problem of edge drop offs as follows:

Unstabilized shoulders frequently are hazardous because the elevation of the shoulder at the pavement edge tends to become one-half to several inches lower than the
pavement ... All types of shoulders should be constructed and maintained flush with the paved surface if they are to fulfill the function for which they are intended... Aside from the inconvenience and car wear that may occur when a motorist drives onto an unstable shoulder, a definite hazard is associated with its use when driven upon at any appreciable speed. Skidding out of control or turning over are not uncommon accidents as a result of loose gravel, sandy, muddy, soft or spongy shoulders.


Similar language is found in the 1954 AASHO Policy on Geometric Design of Rural Highways7, and under the topic, The Forgiving Roadside, in the 1974 AASHTO publication, Highway Design and Operational Practice Related to Highway Safety8.

The Maintenance and Highway Safety Handbook9 published by the Federal Highway Administration in 1977 has the following to say about pavement edge drops (p. 12):

Shoulders are constructed for safety... Proper maintenance is essential to minimize hazardous shoulder conditions usually resulting from the gradual wear of traffic and the presence of water in the shoulder areas... Included are low shoulders, pavement drop-offs... In general, the higher the average
running speed of a road, the smaller the allowable tolerance in edge drop-off.


Pavement edge drops have been a major safety focus of the highway engineering community over the last several years. This emphasis is best documented in the 1983 Traffic Control Devices Handbook10, published by the U.S. Department of Transportation, which states the following with regard to highway construction zones:

Drop-offs should be kept to a minimum in frequency, duration, and depth. When they are inevitable, good judgment should be used to determine the treatment that will be employed. The following items should be considered when developing a traffic control plan for a project that will have a pavement drop-off condition:

  • Where possible, the contract should limit the amount of difference in elevation between adjacent lanes.

     
  • The time that a difference in elevation will be allowed should be limited.

     
  • Signs can be used to advise motorists of the drop-off condition.

     
  • A fillet or wedge of gravel or paving material can be placed.

     
  • Where excessive drop-offs are necessary, it may be possible to close the adjacent lane with appropriate channelizing devices. If the adjacent lane cannot be closed, it may be necessary to install longitudinal roadside barriers such as guardrail or portable concrete barriers.

     
The Roadside Design Guide11 also focuses on pavement edge drops as roadside encumbrances in stating the following:

Pavement edge drop-offs may occur during highway work such as resurfacing or shoulder work. When not properly addressed, drop-offs may lead to an errant vehicle losing control with a high potential for a serious accident.

Desirably, no vertical differential should occur between adjacent lanes or at the edge of pavement. However, when a vertical differential does occur, mitigating measures should be taken. The extent of the measures depend upon:
  • amount of vertical differential;

     
  • longitudinal length of differential;

     
  • location of differential (centerline, lane line and/or edge of pavement);

     
  • duration;

     
  • traffic volume and speed;

     
  • geometrics; and

     
  • relative location of on-coming traffic.

     
Research has found that loss of vehicle control can develop at speeds greater than 30 mph under certain circumstances. . . . This safety problem is minimized where the pavement edge drop-off does not exceed 3 inches in height or the face has a 45-degree slope. Drop-offs immediately adjacent to traffic are not recommended to be left overnight if they are higher than 3 inches (vertical face) or 4.5 inches (45-degree face). To mitigate the pavement drop-off, depending on site specific conditions, one or a combination of the following mitigating measures is recommended:
  • Specify that no vertical drop-off greater than 3 inches in height, or 4.5 inches with a 45-degree or flatter slope, is to be left unprotected overnight.

     
  • Place a wedge of material along the face of the drop-off. The wedge should consist of stable material placed at a 45-degree or flatter slope. Warning signs should be placed in advance and throughout the treatm ent. Pavement markings are useful in delineating the edge of the travel lane.

     
  • Place channelizing devices along the traffic side of the hazard and maintain, if practical, a 3-foot wide buffer between the edge of the travel lane and the drop-off. . . . Warning signs should be placed in advance and throughout the treatment.

     
  • Install portable concrete barriers or other acceptable positive barriers with a buffer between the barrier face and the traveled way.

     
In the early to mid-1980's, many state highway agencies adopted a 3-inch pavement edge height as a maximum tolerable level, based on the report, Pavement Edges and Vehicle Stability - A Basis for Maintenance Guidelines12, by Zimmer and Ivey and on other publications (e.g. The Roadside Design Guide11) that had adopted the recommendations of that study. Later research, however, both has disputed the Zimmer and Ivey results and has provided general guidelines that suggest that a 2-inch criterion is a more appropriate maximum for 55-mph highways. One of these later reports, prepared in response to a mandate by the U.S. Congress, was titled Pavement Edge Drop1 by Olson, Zimmer and Pezoldt. Based on this later work, the Federal Highway Administration and many state highway agencies now have a 1.5- to 2-inch edge height criterion for both routine maintenance and for unprotected pavement edge drops in construction zones.

Pavement edge drops at unpaved shoulders are a recurring nuisance particularly along narrower two-lane roadways with heavy truck traffic. Trucks not only blow away shoulder material during dry weather, but they also frequently disturb shoulder material by running with one wheel of a tandem overhanging the edge, particularly on the inside of highway curves. Then too, unstabilized shoulder material is highly susceptible to rutting by all vehicles during wet weather.

When pavement edge drops of 1.5 inches or more are a frequently recurring problem along a particular roadway, one or more of the following treatments should be used to either replace or supplement the normal maintenance practice of simply adding more shoulder material:
  1. Place low-shoulder warning signs.

     
  2. Add stabilizing material to the shoulder material, which is otherwise susceptible to rutting.

     
  3. Either pave the entire shoulder or pave at least a 2-3 foot strip of shoulder adjacent to the travel lane, particularly along the inside of roadway curves and where pavements are narrower than 22 feet.

     

The most effective way for roadway agencies to mitigate the present and future hazards associated with pavement/shoulder edge drops is to only issue pavement resurfacing contracts where providing a stabilized shoulder flush with the pavement surface is an integral part of the contract. In addition, all resurfacing contracts where the shoulder is unpaved should require a 45° or flatter bevel along the pavement edge.

The appropriate practice for pavement edge drops in construction work zones recognizes that the hazard with any edge drop progressively increases from a position off the shoulder, to the edge of the shoulder, to the edge of the traveled way, to between lanes, to within a travel lane, to in the wheel track of a lane. As this position hazard and the edge-drop height increase, the need for greater traffic control increases as does the expediency for minimizing the exposure to the edge drop through timely construction management. Pavement edge drops placed within a travel lane during resurfacing should generally be discouraged both because of the high exposure to contact and also because of the nearness of potentially conflicting traffic. Positive barriers should be placed where six-inch or higher edge drops are close to traffic, thus eliminating the high probability of vehicle rollover on contact with the edge.

To date, no physical research has been undertaken to address the deleterious effects of pavement edge drop offs for the 70-75 mph speeds of current Interstate highways. The only research of any kind to address speeds higher than 55 mph has been the analytical work by Graham and Glennon2, which suggests a maximum tolerable edge drop of one inch for vertical edges and two inches for rounded edges. These recommendations desirably should be validated with full-scale physical tests.


REFERENCES

1. P. L. Olson, R. A. Zimmer and V. Pezoldt, "Pavement Edge Drop," Transportation Research Board, 1986. Available from www.criterionpress.com.

2. J. L. Graham and John C. Glennon, "Work Zone Design Considerations for Truck Operations and Pavement/Shoulder Drop-Offs," Federal Highway Administration, 1984.

3. John C. Glennon, "Effect of Pavement/Shoulder Drop-Offs on Highway Safety: A Synthesis of Prior Research," Transportation Research Board,1985. Available from www.criterionpress.com.

4. Transportation Research Board, "Designing Safer Roads: Practices for Resurfacing, Restoration and Rehabilitation," Special Report 214, 1987.

5. D. L. Ivey et al., "Safety in Construction Zones Where Pavement Edges and Drop Offs Exist," Transportation Research Record 1163, 1988.

6. American Association of State Highway Officials, "A Policy on Geometric Design of Rural Highways," 1965.

7. American Association of State Highway Officials, "A Policy on Geometric Design of Rural Highways," 1954.

8. Federal Highway Administration. "Handbook of Highway Safety Design and Operating Practices." 1968, 1973 and 1978 eds.

9. Federal Highway Administration,"Maintenance and Highway Safety Handbook," 1977.

10. Federal Highway Administration, "Traffic Control Devices Handbook, 1983, 2001."

11. American Association of State Highway and Transportation Officials, "Roadside Design Guide," 1989, 1996, 2001.

12. R. A. Zimmer and D. L. Ivey, "Pavement Edges and Vehicle Stability: A Basis for Maintenance Guidelines," Texas Transportation Institute, 1982.


OTHER RESOURCES

13. John C. Glennon, "Roadway Safety and Tort Liability", Lawyer and Judges Publishing Co., 2004. Available from www.criterionpress.com.

14. Criterion Press, "Excerpts of Key Documents Related to Pavement Edge Drop-Offs," 2000. Available from www.criterionpress.com.

15. D. L. Ivey and L. I. Griffin, "Driver/Vehicle Reaction to Road Surface Discontinuities and Failures," 16th Congress of the International Federation of the Societies of Automotive Engineers, Tokyo, Japan, 1976.

16. E. F. Nordlin, et al., "The Effect of Longitudinal Edge of Paved Surface Drop-off on Vehicle Stability," California Department of Transportation, 1976.

17. R. H. Klein, W. A. Johnson and H. T. Szostak, "Influence of Road Disturbances on Vehicle Handling," National Highway Traffic Safety Administration, 1977.

18. D. L. Ivey and D. L. Sicking, "The Influence of Pavement Edge and Shoulder Characteristics on Vehicle Handling and Stability," Transportation Research Record 1084, 1986.

19. J. B. Humphreys and J. A. Parham, "The Elimination or Mitigation of Hazards Associated with Pavement Edge Drop-Offs During Roadway Resurfacing," AAA Foundation for Traffic Safety, 1994.

20. L. I. Griffin, "Accident Data Relationships, The Influence of Roadway Surface Discontinuities on Safety: A State-of-the-Art Report," Transportation Research Board, 1984. Available from www.criterionpress.com.

21. D. L. Ivey et al., "Pavement Edges, The Influence of Roadway Surface Discontinuities on Safety: A State-of-the-Art Report," Transportation Research Board, 1988. Available from www.criterionpress.com.


INTERESTING WEB SITES

1. www.ccee.iastate.edu/research/detail.cfm?projectID=255.

2.www.aaafoundation.org/PEDO_RFP.pdf.

3.www.usroads.com/journals/rmej/9911/rm991103.htm.

4. Common Topics in Roadway Defect Cases.

5. Roadway and Traffic Safety References.


About the Author

Dr. John C. Glennon is a traffic engineer with over 45 years experience. He has over 120 publications. He is the author of the book "Roadway Safety and Tort Liability" and is frequently called to testify both about roadway defects and as a crash reconstructionist.

RESUME OF JOHN C. GLENNON




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