Andrea Daly

Roadways classified as state routes in both rural and urban settings are tree-lined, creating a canopy over the pavement surface. Although trees are beneficial to the environment, they pose a hazard to road safety in a variety of ways. For example, they offer reduced skid resistance due to fallen leaves; restrict direct sunlight on pavement surface, causing the formation of black ice and fog; and increase the risk of entire trees/branches/fruits falling on passing vehicles or blocking traffic lanes. State agencies have undertaken the task of eliminating tree canopies, inviting widespread criticism. Currently, there is a gap in the literature about the impact of tree canopy on traffic safety in the form of scientific research. This paper addresses this gap by presenting insight into the benefits of tree-trimming and/or removal operations along state routes in Ohio. Adopting the Empirical Bayesian approach, this study evaluates crashes to quantify the safety benefits. Additionally, a surrogate safety assessment was conducted to evaluate driver behaviors in the presence/absence of tree canopy. The results indicated that roadside tree trimming and pruning showed site-specific safety benefits at most locations, though no consistent project-level crash reduction or conclusive effects on surrogate safety measures, such as reduced speeds and hard braking, were observed.

Road managers require convenient, inexpensive methods to quickly assess pavement conditions over large areas. New imaging technologies potentially offer a substantial improvement over current methods. In this study, two hand-portable methods based on widely available consumer-grade technology were tested across a range of pavement conditions. To place the proposed methods in a familiar context, data were compared with the mean texture depth (MTD) calculated by the widely used volumetric method. MTD provided the clearest distinction between experimental roughness classes. Spatial pattern analysis (fractal dimension) of digital images discriminated between light, moderate and severe deterioration but failed to make fine distinctions between degrees of moderate wear. Three-dimensional models constructed by stereoscopic infrared scanning distinguished between all but the smoothest texture classes. It appears that both the proposed methods have potential for accurate, quick and convenient assessment of pavement macrotexture at little expense, although some judgement must be used in their application.