Does Roadside Vegetation Affect Driving Performance? Driving Simulator Study on the Effects of Trees on Driver’s Speed and Lateral Position

This study investigated the effects of roadside vegetation on driving performance on a two-lane rural road. Forty-four participants drove along seven different roadside tree configurations implemented in a driving simulator. Configurations were characterized by two offsets of trees from the road edge (1.5 m and 4.0 m) and three spacings between trees (10.0 m, 17.5 m and 25.0 m) located on the roadside of a 6.0 m wide two-lane rural road. One additional configuration, without trees, was used as the baseline condition. The investigation was developed over five geometric elements: sharp/gentle, left/right curves and tangent. The driver’s lateral position and speed were subsequently collected. Compared to the baseline condition, it was found that when trees were close to the road edge drivers tended to significantly decrease their speed and moved towards the centerline of the road. On the contrary, when the offset of trees increased, drivers adopted higher speeds, increasing the distance from the road edge but with a lower left lateral displacement. This occurred along all five geometries, especially on sharp curves. Tree spacing did not affect the driver’s speed but significantly influence the lateral position: drivers moved further away from the road edge when tree spacing decreased. The results demonstrate that drivers balance the useful guidance information that roadside trees provide with the risk associated with their presence: when trees are far, the sense of guidance is predominant and drivers adopt higher speeds; when trees are close, they are seen as a risk by drivers who consequently slow down and move further away from them. Such driving behaviour has direct impacts on the safety implications of roadside trees which are discussed in this paper.