Cost-Effective Safety Treatment of Trees on Low-Volume Rural Roads

Generally, trees are naturally occurring fixed objects that are found along many roadways and that potentially pose safety risks to errant motorists. Unfortunately, trees have been responsible for numerous fatal and serious injury crashes during run-off-road events. This study included an incremental benefit-to-cost (B-C) analysis that used the Roadside Safety Analysis Program to investigate the efficacy of safety treatment alternatives for trees on roadways with volumes of less than 500 vehicles per day (vpd) and speed limits of 55 mph (88.5 km/h) or greater. The study was based on a parametric analysis of site characteristics from a field survey in Kansas. It used four tree groupings, three tree diameters, and four lateral offsets from the roadway to configure 120 scenarios. Three safety treatment methods were considered: (a) a do-nothing option representing the baseline condition; (b) tree removal, with cost estimates coming from reliable sources; and (c) a crashworthy guardrail system. For various reasons, the guardrail system was no more cost-effective than the do-nothing or tree removal options. B-C ratios were used to recommend tree removal on the basis of several pertinent variables. In all cases, B-C ratios for tree removal were never less than 1.0, which indicated limited justification for keeping the trees. Tree removal was considered the safest and primary alternative when trees were far from other fixed obstacles. Because these guidelines are based solely on B-C analyses, the road designer or engineer is encouraged to use them as a foundation for making safety improvements but also to consider site-specific investigation and analysis.