Accident Rate and Severity Ratio Calculator: Accidents per 100 Million Vehicle-Miles and per Million Entering Vehicles
Standard highway safety measures from a count of accidents and traffic.
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How this calculator works
Choose the accident rate of a length of road, the accident rate at an intersection or the severity ratio.
Enter the number of accidents, the average daily traffic, the number of years and, for a road, its length.
The calculator gives the rate, the accidents per year, the accidents in another period and the years to reach a number of accidents.
Formulas used
- R = \frac{A \times 100\,000\,000}{ADT \times N \times 365 \times L}
- R = \frac{A \times 1\,000\,000}{ADT \times N \times 365}
- SR = \frac{f + i}{f + i + p}
Worked example
With the default numbers in the calculator above:
- Accident rate of the road (per 100 million vehicle-miles): 79515.27 accidents per 100 million vehicle-miles
- Accidents per year: 844.57
- Accidents in the other period at the same rate: 4222.86
- Years to reach that number of accidents: 1.18 years
Change the numbers in the calculator to see the full working, every step, for your own case.
A second worked example
A different case, with these numbers: What do you want to find? Accident rate at an intersection; Number of accidents in the period 5892; Average daily traffic (vehicles per day; for an intersection, all legs together) 476; Length of the period 6 years.
- Accident rate at the intersection (per million entering vehicles): 5652.12 accidents per million entering vehicles
- Accidents per year: 982.00
- Accidents in the other period at the same rate: 4910.00
- Years to reach that number of accidents: 1.02 years
Common questions
What is the accident rate per 100 million vehicle-miles?
The number of accidents on a length of road, divided by the vehicle-miles travelled on it in the period, scaled to 100 million. It lets roads of different lengths and traffic be compared.
What is the accident rate at an intersection?
The number of accidents divided by the number of vehicles that entered the intersection from all legs, scaled to one million entering vehicles.
What is the severity ratio?
The fatal plus injury accidents divided by all accidents (fatal, injury and property damage only). It is the share of accidents that caused a death or an injury.
Related calculators
Got a question about this? Ask an engineering question and get a plain-language answer with references.
Further reading: Any highway safety or traffic engineering text (accident rates and severity ratio).
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