Safety Index for evaluation of urban roundabouts

Area of Science:

• Road Safety Engineering
• Traffic Engineering
• Transportation Safety

Background:

• Growing EU requirements for network-wide road safety assessments (EU Directive 2019/1936).
• Need for reliable procedures to assess inherent safety of road network elements.
• Focus on identifying risk factors in road characteristics for safety evaluations.

Purpose of the Study:

• Develop and validate a Safety Index (SI) for evaluating urban roundabouts.
• Assess safety at both the roundabout and approach levels.
• Identify key safety issues contributing to crash risk for targeted interventions.

Main Methods:

• Formulated a Safety Index (SI) combining user exposure and risk factors.
• Incorporated 33 detailed and 5 general safety issues into the Risk Index calculation.
• Validated the SI procedure on 50 urban roundabouts in Rome, Italy (179 approaches).
• Compared SI scores with Empirical Bayes (EB) crash estimates.

Main Results:

• Highly significant correlation between SI scores and EB crash estimates at roundabout level (R²=0.85) and approach level (R²=0.56).
• Spearman's rank-correlation analysis confirmed agreement between SI and EB rankings at the 99.9% confidence level (ρs=0.80 roundabout, ρs=0.70 approach).
• The SI effectively detects safety issues and ranks their contribution to crash risk.

Conclusions:

• The developed Safety Index (SI) is a reliable tool for network-wide road safety assessment of urban roundabouts.
• The SI procedure accurately identifies and ranks safety issues, facilitating effective safety measure implementation.
• The validation demonstrates the SI's utility in improving road infrastructure safety management.