Rural two-lane highways constitute a majority of the mileage of public roads in the United States. Rural two-lane highways have 40% of the total travel but experience 60% of the highway fatalities. During the 1980's and 1990's, research was carried out in the United States to quantify the safety and operational effects of various key geometric design features including: lane and shoulder widths; roadside safety; horizontal and vertical alignment; sight distance; rural intersections. Aggregating this research has resulted in a compendium of the safety benefits of certain geometric design features for two-lane rural highways which links design standards and safety. This aggregated research information provides a basis for software analysis of proposed highway designs to assess their expected safety performance as well as the traditional capacity performance. Application of these benefits and effects achieves a numerical methodology for safety in a similar manner as is commonly carried out for capacity. This compendium of the safety and operational benefits of highway design features is being deployed in the United States to state highway departments in advance of the release of the software based analysis system. For the covering abstract of this conference see ITRD number E211395.

This report presents an algorithm for predicting the safety performance of a rural two-lane highway. The accident prediction algorithm consists of base models and accident modification factors for both roadway segments and at-grade intersections on rural two-lane highways. The base models provide an estimate of the safety performance of a roadway or intersection for a set of assumed nominal or base conditions. The accident modification factors adjust the base model predictions to account for the effects on safety for roadway segments of lane width, shoulder width, shoulder type, horizontal curves, grades, driveway density, two-way left-turn lanes, passing lanes, roadside design and the effects on safety for at-grade intersections of skew angle, traffic control, exclusive left- and right-turn lanes, sight distance, and driveways. The accident prediction algorithm is intended for application by highway agencies to estimate the safety performance of an existing or proposed roadway. The algorithm can be used to compare the anticipated safety performance of two or more geometric alternatives for a proposed highway improvement. The accident prediction algorithm includes a calibration procedure that can be used to adapt the predicted results to the safety conditions encountered by any particular highway agency on rural two-lane highways. The algorithm also includes an Empirical Bayes procedure that can be applied to utilize the safety predictions provided by the algorithm together with actual site-specific accident history data.

When accidents happen, drivers are blamed for the mishap. When drivers consistently fail at certain locations, it then becomes obvious that the problem lies not with them, but with the geometry of the road itself. Because accidents are not evenly distributed throughout the road network, locations with high accident rates are a clear indication that there are other factors involved, besides driver error, which are characterized by the road itself. In most countries, two-lane rural roads make up about 90 percent of rural networks and they account for over 60 percent of highway fatalities worldwide, approximately 500,000 people per year. The methodology described in this book will support the achievement of quantified measures of: design consistency; operating speed consistency; and, driving dynamic consistency. The safety criteria are then combined into an overall safety module for a simplified general overview of the safety evaluation process. The authors also encourage the coordination of safety concerns with important economic, environmental and aesthetic considerations. This book will be an invaluable aid to educators, students, consultants, highway engineers and administrators, as well as scientists in the fields of highway design and traffic safety engineering.