Assessing the Consequences of Tree Failure

Abstract Arborists, urban foresters, and other tree care professionals commonly use visual risk assessment methods to evaluate tree safety. Most methods used by professionals have three main inputs: likelihood of impact, likelihood of failure, and consequences of failure. In assessing the latter, the size of the tree part expected to fail and its fall distance are key aspects of determining potential damage to nearby targets. As such, accurately rating consequences of failure requires both an ability to estimate sizes and fall distances from afar and an ability to interpret how this information relates to target impact. In this study, we assessed how arborists with different levels of experience and training visually rated consequences of failure for human and vehicular targets given their visual estimations of limb size and fall distance. A total of 87 arborists participated in this study designed to assess sources of variation in consequences of failure ratings. Findings from this work suggest that arborists are fairly accurate in estimating branch size based on diameter. When asked to rate consequences of failure for various diameter branches, the majority of arborists selected “significant” to “severe” for stems equal to or greater than 29.2 cm (11.5 inches) when the target was a pedestrian. For vehicular targets, this threshold was 69.2 cm (27.2 inches).