In the development of roadside safety improvement programs, many types of obstacles have been identified as being hazardous. However, little attention has been given to the hazard of driveway slopes along noncontrolled and limited-access roadways. It was the purpose of this study to assess the hazard posed by such driveway slopes and to determine the cost-effectiveness of flattening them. The degree of hazard was measured in terms of the expected number of injury (fatal or nonfatal) accidents per year resulting from a vehicle traversing the slope. The probability of injury in a run-off-the-road encroachment of a driveway slope, which was used to compute the degree of hazard, was derived from severity indexes computed from results obtained from the highway vehicle object simulation model to simulate a standard-sized automobile traversing driveway slopes under encroachment conditions of 91-km/h (55-mph) speed at 10 deg encroachment angle in a free-wheeling steering mode. The results of this study indicate that (a) driveways are a roadside hazard,(b) the most cost-effective driveway slope design standard is 8: 1, and (c) flattening to an 8: 1 slope is the most cost-effective driveway slope improvement. The cost-effectiveness methodology used in this study provides a common basis for comparing driveway slope improvements with other types of improvements in the management of roadside safety improvement programs.