Start date: 06/01/97
End date: 08/31/02
About the research
Pavement and shoulder edge drop-offs commonly occur in work zones as the result of overlays, pavement replacement, or shoulder construction. The depth of these elevation differentials can vary from approximately one inch when a flexible pavement overlay is applied to several feet where major reconstruction is undertaken. The potential hazards associated with pavement edge differentials depend on several factors including depth of the drop-off, shape of the pavement edge, distance from traveled way, vehicle speed, traffic mix, volume, and other factors. This research was undertaken to review current practices in other states for temporary traffic control strategies addressing lane edge differentials and to analyze crash data and resultant litigation related to edge drop-offs. An objective was to identify cost-effective practices that would minimize the potential for and impacts of edge drop crashes in work zones. Considerable variation in addressing temporary traffic control in work zones with edge drop-off exposure was found among the states surveyed. Some states base traffic control plans on only one factorthe depth of the drop-off. Other states consider additional issues such as the expected duration of exposure, posted speed limit, average annual daily traffic, and the lateral distance from the traveled way to the drop-off. All of the states surveyed indicated that edge drop-off treatment is initiated before a depth of three inches is encountered. The elevation differential warranting the use of temporary traffic barriers varies from two inches to two feet, depending on individual state policy. Crashes related to pavement edge drop-offs in work zones do not commonly occur in the state of Iowa, but some have resulted in significant tort claims and settlements. No major deficiencies were identified in the current temporary traffic control procedures used by the Iowa Department of Transportation for work zones that include pavement edge differentials. Prudent drivers apparently travel through Iowa work zones without excessive difficulty or significant speed reduction. The use of a benefit/cost analysis may provide guidance in selection of an appropriate mitigation and protection of edge drop-off conditions. While not as compelling as the need for adequate safety, economic analysis does offer an objective, quantifiable rationale for preliminary decision making for selection of temporary traffic control. Development and adoption of guidelines for design of appropriate traffic control for work zones that include edge drop-off exposure, particularly identifying effective use of temporary barrier rail, may be beneficial in Iowa.
Report: http://www.intrans.iastate.edu/reports/dropoff.pdf (887KB pdf) August 2002
Sponsor(s): Iowa Department of Transportation