Proposed Heavy Vehicle Encroachment Adjustment Factor

Encroachment probability models such as the Roadside Safety Analysis Program (RSAP) have traditionally assumed that heavy vehicles and passenger vehicles share the same encroachment characteristics. This assumption was reviewed in developing bridge railing selection guidelines in NCHRP 22-12(03), where an examination of a specific highway and a national sample of data indicated that trucks encroached at a different rate than passenger vehicles. This paper describes the development of a new vehicle-type encroachment adjustment factor (EAF). The results confirmed previous findings, but this analysis controlled for traffic volumes, highway type, percentage of heavy vehicles [i.e., percentage of trucks (PT)], and segment length. The result was a more robust model that was valid over a wider range of average annual daily traffic and PTs. The large data set included 635,464 segments of data from the states of Ohio and Washington. The proposed EAF was recommended for inclusion in RSAPv3. Ideally, encroachment data would be collected for heavy vehicles to determine the frequency of heavy vehicles encroaching onto the roadside and the trajectories heavy vehicles took during encroachment, but this process proved to be financially challenging. The study used crash data to carry out a comprehensive analysis of traffic volume, heavy vehicle mix, highway type, and segment length. A vehicle-type EAF was developed for divided and undivided roadways. The results provided some indication of how best to incorporate heavy vehicles in the encroachment probability model used in RSAP.