Does Roadside Vegetation Affect Driving Performance?: Driving Simulator Study on the Effects of Trees on Drivers' Speed and Lateral Position

Calvi, Alessandro · 2015 · Crossref

DOI: 10.3141/2518-01

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Summary

This study investigates how roadside vegetation, specifically trees, influences driving performance on narrow two-lane rural roads. The research addresses the critical safety issue of single-vehicle accidents, which account for a disproportionate number of traffic fatalities, particularly on rural roads where "unforgiving" roadside designs exacerbate crash severity. While previous literature offered conflicting findings regarding whether trees act as guidance cues or hazards, this study aims to clarify how specific tree configurations—defined by offset from the road edge and spacing between trees—affect driver speed and lateral position across various road geometries. The experiment utilized a fixed-based driving simulator at Roma Tre University, involving 38 participants with significant rural driving experience. Researchers manipulated three independent variables: roadway geometry (sharp and gentle left/right curves, and tangents), tree offset from the road edge (1.5 m or 4.0 m), and tree spacing (10.0 m, 17.5 m, or 25.0 m). A baseline condition with no trees was included for comparison. The dependent measures were average driving speed and lateral position, defined as the distance from the vehicle center to the road centerline. Statistical analysis using ANOVA with repeated measures evaluated the effects of these variables on driving behavior. The results revealed that tree offset significantly impacted both speed and lateral position, whereas tree spacing only affected lateral position. When trees were located close to the road edge (1.5 m offset), drivers significantly decreased their speeds and shifted their lateral position toward the road centerline, treating the trees as hazards. Conversely, when trees were further away (4.0 m offset), drivers adopted higher speeds than in the baseline condition, perceiving the trees primarily as guidance features that improved road edge delineation. This guidance effect was most pronounced on gentle curves and tangents, while the hazard perception dominated on sharp curves. Regarding spacing, drivers moved further away from the road edge as tree spacing decreased, indicating that denser vegetation prompted greater lateral avoidance, though this did not significantly alter speed. The study concludes that drivers balance the useful guidance information provided by roadside trees against the risk associated with their presence. These findings have significant implications for road safety and design. Trees placed far from the road edge may inadvertently encourage excessive speeds, increasing the risk of run-off-road accidents on complex geometries like sharp curves. Furthermore, the tendency to move toward the road centerline to avoid nearby trees increases the risk of head-on collisions, particularly on sharp left curves where drivers cut the trajectory. The authors suggest that trees close to the road edge could serve as effective speed-reducing measures if protected by barriers, but emphasize the need for careful design to mitigate lateral positioning risks. Future research is recommended to explore alternative vegetation types, such as bushes, which might provide guidance without the same collision severity.

Key finding

Drivers significantly decrease speed and shift laterally toward the road centerline when roadside trees are close to the road edge, but increase speed when trees are further away, while closer tree spacing consistently pushes drivers away from the road edge regardless of offset.

Methodology

simulator

Sample size: 44

Provenance

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Summary generated by qwen3.6-27b-prismaquant on 2026-06-10; verification: verified.

Topics

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• perceptual countermeasures
• road geometry
• external distraction

Information type

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• Empirical Findings: behavioral performance data