Below is the uncorrected machine-read text of this chapter, intended to provide our own search engines and external engines with highly rich, chapter-representative searchable text of each book. Because it is UNCORRECTED material, please consider the following text as a useful but insufficient proxy for the authoritative book pages.
HFG RURAL ENVIRONMENTS Version 1.0 COUNTERMEASURES FOR PAVEMENT/SHOULDER DROP-OFFS Introduction A shoulder is a portion of the roadway contiguous with the traveled way for accommodation of stopped vehicles, for emergency use, and for lateral support of the sub-base, base, and surface courses. The roadway shoulder has been recognized as desirable ever since engineers began paving roadways. However, the width, uniformity, and stability of roadway shoulders have varied greatly from roadway to roadway and along different sections of the same roadway (1). Shoulders on rural roadways serve as structural support for the surfacing and additional width for the traveled way. Shoulder drop-offs occur when there is a difference in height (ranging from a fraction of an inch to several inches) between the pavement surface and the roadside surface (2). This height difference typically arises from tire rutting erosion, excessive wear, or resurfacing. The primary concern related to drop -offs is that if they are too high, then it can pose a crash risk if a vehicle drifts outside the road and has a wheel go over the drop-off. Design Guidelines Vertical or near-vertical shoulder drop-off heights that exceed the indicated table values warrant consideration for drop-off treatment or traffic control (in work zones; adapted from Graham & Glennon (3)). VERTICAL DROP-OFF HEIGHT WARRANTING TRAFFIC CONTROL FOR VARIOUS LANE WIDTHS Speed Drop-off Height (mi/h) 12-ft Lane Width 11-ft Lane Width 10-ft Lane Width 9-ft Lane Width 30 3 in. 3 in. 3 in. 2 in. 35 3 in. 3 in. 2 in. 1 in. 40 3 in. 2 in. 1 in. 1 in. 45 2 in. 1 in. 1 in. 1 in. 50 1 in. 1 in. 1 in. 1 in. Based Primarily on Based Equally on Expert Judgment Based Primarily on Expert Judgment and Empirical Data Empirical Data EXAMPLE OF SAFETY EDGE RECOMMENDED BY THE FHWA (4 ) 16-4
HFG RURAL ENVIRONMENTS Version 1.0 Discussion A primary safety concern related to drop-off from the perspective of driver performance occurs when a vehicle leaves the lane and has a tire go over the drop-off (typically the front right tire). This event is often surprising and unfamiliar for most drivers, and a drop-off can interfere with their ability to return the vehicle safely to the lane. In particular, if drivers try to return to the lane at high speed and at low steering angles, their tire can "scrub" against the drop-off edge, impeding their return. A common response is to increase the steering angle towards the lane, which can lead to an abrupt change in heading once the drop-off is overcome. In severe cases, this response can result in a vehicle swerving out of the lane into the opposite-direction travel lane, putting the driver at risk of a collision with an oncoming vehicle. Motorcyclists can also have difficulties traversing drop-offs, although the vehicle control issues are somewhat different. Crash data analyses suggest that the overall frequency of crashes "probably" or "possibly" related to high drop-off is relatively low (less than 3% of rural road crashes for related road types), but that these crashes tend to result in a greater proportion of fatalities or injuries than typical rural road crashes (4). The guideline information is primarily based on an analysis of drop-offs for work zones (3). The original source table also contained drop-off height thresholds that were higher than 3 in., but these were changed in the current guideline to reflect a more conservative assessment of other related driver performance data on driver encounters with drop-offs of various heights (4). Note that the recommendation only represents general guidance related to driver performance; other sources--such as the Roadside Design Guide (5)--recommend that vertical drop-offs with differentials of 2 in. or more should be avoided. What the guideline table is intended to convey is that vertical drop- off heights that exceed the listed values are more likely to be associated with increased difficulty for drivers trying to recover in a controlled manner if one of their tires go over the drop-off edge. Another design aspect related to drop-offs that affects driver performance is the shape of drop-off. In particular, safe return to the lane is significantly more successful if a tire had to overcome a drop-off with a slope of 45° or shallower. The figure accompanying the guideline illustrates the relative "safety" of three drop-off geometries. Lane recovery with a sloped or filleted drop-off is significantly better than a straight vertical or curved drop-off. Moreover, the effectiveness of sloped drop-offs persists at higher speeds and at higher drop-off heights (4). Design Issues There are several accepted approaches for addressing drop-offs that are too high. For example, in work zones MUTCD warning signs for edge drop-off can notify users of present drop-off conditions. The application of a wedge-shaped asphalt material called "Safety Edge" is another possible countermeasure (see figure on previous page). When placed between the roadway and the shoulder, the material can help drivers recover from the shoulder to the driving surface. The asphalt material needs to be compacted to increase strength, otherwise the material will break apart over time due to forces and runoff water. Graham, Richard, and Harwood (6) found the results of empirical Bayes and cross-sectional analysis of sites paved with and without Safety Edge reveal that the material has a net positive effect on the safety of rural highways. Humphreys and Parham (1) found that the shoulder is best resurfaced when the roadway is resurfaced so that shoulder drop-off does not form. They also recommend that the contractor, in areas where road-resurfacing contracts must be bid separately, should be required to provide a 45° angle fillet along the edge of the roadway as part of the scope of work. Cross References Design Consistency in Rural Driving, 16-8 Key References 1. Humphreys, J.B., and Parham, J.A. (1994). The Elimination or Mitigation of Hazards Associated with Pavement Edge Drop-offs During Roadway Resurfacing. Washington, DC.: AAA Foundation for Traffic Safety. 2. Fitzpatrick, K., Parham, A.H., and Brewer, M.A. (2002). Treatments for Crashes on Rural Two-Lane Highways in Texas (FHWA/TX- 02/4048-2). College Station: Texas Transportation Institute. 3. Graham, J.L., and Glennon, J.C. (1984). Work Zone Design Considerations for Truck Operations and Pavement/Shoulder Drop-offs. Washington, DC: FHWA. 4. Hallmark, S.L., Veneziano, D., McDonald, T., Graham, J., Bauer, K.M., Patel, R., and Council, F.M. (2006). Safety Impacts of Pavement Edge Drop-Offs. Washington, D.C.: AAA Foundation for Traffic Safety. 5. AASHTO (2002). Roadside Design Guide. Washington, DC. 6. Graham, J.L., Richard, K.R., and Harwood, D.W. (2009). Safety Evaluation of Safety Edge Treatment--Year 2, Interim Report. Kansas City, MO: Midwest Research Institute. 16-5
