Properties of Encroachments and the Associated Roadway and Roadside Environment in the Second Strategic Highway Research Program

An encroachment is defined as a vehicle departing the lanes of the traveled way and entering the roadside or median. Encroachments have been previously characterized by Hutchinson and Kennedy and by Cooper, in studies conducted in 1964 and 1978, respectively. Both studies are more than 40 years old, and both roadways and vehicles have changed significantly since data collection ended, presenting the need for an updated encroachment study. This study identified and characterized encroachment events in the Second Strategic Highway Research Program Naturalistic Driving Study (SHRP 2 NDS). Then, we compared the statistical distributions of roadway and roadside environmental factors weighted by their occurrence in SHRP 2 NDS normal driving and during these encroachment events. This weighting represented the typical infrastructure SHRP 2 vehicles encountered, as opposed to the infrastructure installed. Segments of roadway encountered more frequently or encroachment areas represented more frequently were weighted more heavily. A total of 863,280 SHRP 2 trips containing normal driving and road-departure events were selected from the SHRP 2 dataset which included 215,993 h of driving. A total of 278,415 departure events were found. Encroachment events were similar to normal driving in roadway grade, shoulder width, and lane width. Encroachment events occurred more frequently on roadway sections with depressed medians, one or two lanes in the travel direction, smaller horizontal curve radii, or rumble strips present.