Note:

Expected Accident Rate (EAR) predicts post-improvement safety performance by accounting for planned accident reductions relative to traffic exposure. It helps transportation planners assess the effectiveness of road safety measures before implementation. By incorporating historical data, traffic volume, and proposed interventions, EAR provides a realistic estimate of future accident rates. This metric is essential for justifying infrastructure upgrades, optimizing resource allocation, and enhancing overall road safety.

Key Parameters:

• Total Accidents (B) - This represents the total number of accidents recorded annually for the selected road segment or intersections. It serves as the baseline for calculating improvements.
• Reduction Factor - The estimated percentage decrease in accidents due to safety measures. For example, if a new traffic signal is expected to reduce accidents by 30%, enter "30" (not 0.30). The reduction factor helps determine how effective the safety intervention will be.
• Average Daily Traffic (ADT) - The total number of vehicles passing through the road segment or intersection per day, measured in thousands. For example, an ADT of 15 means 15,000 vehicles per day. Higher ADT values indicate busier roads, where accident rates may be higher.
• Number of Locations/Miles (N) - The total length of the road segment (in miles) or the number of intersections being analyzed. This helps normalize the accident rate per unit distance or per intersection for better comparison across different locations.
• 0.365 is used to convert daily traffic volume (ADT) into an annual scale by approximating the number of days in a year when traffic is active.

Real-Life Applications:

• Road Safety Improvements - Projecting benefits of new designs
• Traffic Engineering - Comparing alternative treatments
• Grant Applications - Quantifying expected safety benefits
• Public Health Planning - Estimating injury reduction
• Insurance Underwriting - Assessing risk mitigation

Why Include in Safety Calculator?

Essential for evaluating proposed safety projects, justifying infrastructure investments, and setting measurable safety performance targets.

Lessons from History:

• Roundabout Conversions typically show 35-50% reduction in severe crashes
• Overestimated reductions have led to underfunded safety projects

Industry Standards:

FHWA's Toolkit for Addressing Safety on Urban Streets provides reduction factors. AASHTO's Highway Safety Manual gives predictive methods.

Conclusion:

The EAR calculation transforms safety engineering predictions into quantifiable metrics, enabling data-driven decisions about traffic safety investments and policy changes.