A6.7 - Adjusting accident costs to reflect mean speeds
- A6.1- Accident costs
- A6.2 - Choosing to undertake an accident analysis
- A6.3 - Applying the analysis methods
- A6.4 - Accident trends
- A6.5 - Typical injury accident rates and prediction models
- A6.6 - Typical accident reduction factors
- A6.7 - Adjusting accident costs to reflect mean speeds
- A6.8 - Worked example of accident procedures
- A6.9 - Tables
- A6.10 - References
A6.7 - Adjusting accident costs to reflect mean speeds
Effect of speed on accident costs
Evidence indicates that injuries per accident and injury severity increase linearly with speed. To account for this in an accident analysis, the accident costs for the do minimum and the option(s) are calculated using mean traffic speeds.
Adjusting accident costs by movement and vehicle involvement
Accident costs by movement and vehicle involvement for use in Method A are provided for 50 km/h speed limits and 100 km/h speed limits in table A6.21(a) to (h).
Where the mean speed of the do minimum and/or project options differ from these speeds, the accident costs are adjusted using the following formula:
CV = C50 + (C100 - C50)(V - 50)/50
where: CV is the cost of accidents for the mean speed V
C50 is the cost of accidents in 50 km/h speed limit areas
C100 is the cost of accidents in 100 km/h speed limit area
V is the mean speed of traffic in km/h
Adjusting reported injury accident costs
Costs per reported injury accident for use in Method B or C are provided for 50, 70 and 100 km/h speed limits in table A6.22.
Where the mean speed of the do minimum and/or project options differ from these speeds, the accident costs are adjusted using the one of the following formulae:
for 50 < V < 70 km/h: CV = C50 + (C70 × C50)(V - 50)/20
for 70 < V < 100 km/h: CV = C70 + (C100- C70)(V - 70)/30
where: CV is the cost of accidents for the mean speed V
C50 is the cost of accidents in 50 km/h speed limit areas
C70 is the cost of accidents in 70 km/h speed limit areas
C100 is the cost of accidents in 100 km/h speed limit area
V is the mean speed of traffic in km/h
Calculation of mean speed
If the road section has a design speed based on the 85th percentile speed then to convert the design speed to the mean speed use the approximation of dividing the 85th percentile speed by 1.13 (or multiplying by 0.885) and round the result to the nearest whole kilometre per hour.
Mean speed should be established over a section length of at least 1 kilometre.
