Pavement edge drop-off is the vertical difference in elevation between the paved roadway and the adjacent ground. It is a serious safety concern for vehicles that goes off the track and encounters a drop-off. The errant vehicles, in order to restore their position back on the paved road, exert a greater amount of force which may result in loss of control for the driver. This may indicate an increase in the possibility of lane departure crashes, rollovers or head on collisions. According to an estimation by the Federal Highway Administration about 11,000 people suffers from injuries and about 160 people lose their lives each year in crashes related to unsafe pavement edges in the United States. Safety Edge, on the other hand, is a design feature that creates a fillet along the edge of the pavement of the roadway that allows drivers, who drift off roadways, to return safely to the roads. This study intended to conduct a safety evaluation of road segments provided with Safety Edge in Iowa. Thus a before and after crash analysis was conducted to estimate any reduction in crashes in the after period of installation. This research also looked into the road and traffic characteristics that significantly affected the crashes on road segments provided with Safety Edge.

A total length of 483 miles of roadway segments installed with Safety Edge was identified all over Iowa. Roadway, traffic, lane characteristics and crashes on the treatment segments for the study period of eleven years from 2004 to 2011 was obtained from Iowa Department of Transportation. A Preliminary before and after crash analysis for all types of crashes showed a 50% reduction in all types of fatal crashes, 18.5% reduction in all types of property damage only (PDO) crashes and an overall decrease of 19% for all types of total crashes. A preliminary before and after crash analysis for target crashes showed a 75% reduction in Target fatal crashes, 1% increase in target PDO crashes and overall 17% reduction in total target crashes.

The crash data which is a form of count data was analyzed using negative binomial regression. Positive safety impact of installation of Safety Edges was observed for almost all the statistical models (except for property damage only target crashes), as the crashes in the after period was observed to be less than that of the before period. Both scenarios of all types of crashes and target crashes were considered separately in the study. The variables that significantly affected the different crash models were average annual daily traffic (AADT), shoulder width, Rural/Urban indicator, and surface width. Negative Binomial Models for All types of KABCO crashes (all crash severity levels taken together) showed 21% reduction in crashes in the after period. The percentage reduction of all types of injury (KABC) crashes was 20%. For all types of PDO crashes the reduction was seen to be again 20%. Negative Binomial Models for target crashes showed 16.3% reduction in target KABCO crashes and 2.4% increase in target PDO crashes, along with 21% reduction in all types of KABCO crashes, and 20 % reduction in all types of PDO crashes. The results indicated that Safety Edge installation may also be able to reduce the severity of a crash.