Relationship between Side Slope Conditions and Collision Records in Washington State

Design guidelines for road side slopes in Washington State follow generalized methods and require cost-benefit
analysis. Prior to this research project, the effects of current methodology had not previously been evaluated.
This report outlines the results of research on the effectiveness of slope flattening in reducing the number
and severity of collisions on Washington State highways in rural areas. A before and after study was
performed by analyzing 3R and 2R projects in Washington State which included side slope flattening projects
that were accomplished from 1986 through 1991.

The study shows that side slope flattening reduces both the number and severity of collisions when
compared to highway sections without side slope flattening. Even when including the effects of various
nonstructural initiatives which have helped reduce collisions, slope flattened sections exhibited lower collision
rates. The research lends credence to current design practice which utilizes benefit-cost analysis when prioritizing
roadside safety improvement projects.

Publication Date: 
Wednesday, December 4, 1996
Publication Number: 
WA-RD 425.1
Last modified: 
08/04/2017 - 16:13
Chris Allaire, Dan Ahner, Manuel Abarca, Paul Adgar, Sarady Long.
Saint Martin's College. Engineering Division.
Number of Pages: 
Highway design, Slopes, Crash rates, Ran off road crashes, Rural highways, Before and after studies, Roadside improvement, Safety, Cost effectiveness, Evaluation and assessment, Benefit cost analysis, Recommendations.