Reexamination of Roadside Encroachment Data

Concerns about the Hutchinson and Kennedy (H&K) encroachment data used as the basis for AASHTO’s Roadside Design Guide (RDG) procedures for computing guardrail runout length have led to proposed revised guidelines based on more recent data collected by Cooper in Canada. The revised guidelines result in guardrail lengths that are approximately 35 percent shorter. Data from the original H&K and Cooper studies were analyzed to determine why the two studies seem to have inconsistent findings. The concerns about the H&K data expressed by the authors who proposed the revised guidelines were found to be invalid. However, team-by-team comparisons of the Cooper data revealed inconsistencies in encroachment rates, lengths, and departure angles for highways with similar speed limits. Documented data collection problems and unanswerable questions about the methodology used to measure encroachment lengths and departure angles make it impossible to validate the accuracy of the Cooper data. Adjustments were made to the Cooper data to compensate for possible data irregularities to examine the consistency between them and the H&K data. No evidence indicating that the encroachment lengths reported by H&K are inconsistently high compared with the encroachment lengths reported by Cooper was found. The findings suggest that reducing guardrail runout lengths from current RDG guidelines for highways with high speed limits [112 km/h (70 mph)] may not be prudent.