A6.8 - Worked example of accident procedures
- 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
A6.8 - Worked example of accident procedures
Introduction
This section provides a worked example using Methods B and C.
Do minimum accident costs
A straight and flat 3.3 km section of near rural road in a 100 km/h area is identified as having a high incident of loss of control accidents. This section of road has two 3.5 m lanes and no sealed shoulder. The AADT is 2,800 with a traffic growth rate of 4 percent. Nine injury accidents were recorded in CAS for the previous five years. Two of these were serious injury accidents.
The option is to widen the seal to 9 m in total: two 3.5 m lanes and 1 m wide sealed shoulders. Time zero is 2006.
The weighted accident procedure is used as there are less than three injury accidents, or one serious or fatal accident, per kilometre in the previous five years. Exposure-based accident prediction equations are available for the do minimum and option (appendix A6.5).
The proposed improvement (seal widening) is not considered a fundamental change, and hence the accident history is still relevant in calculating the site specific accident rate and costs.
Do minimum
Site specific accident rate AS
AS = 9 injury accidents / 5 years for the site history × 1.10
where: 1.10 is the accident trend adjustment factor from table A6.1(a)
AS = 9 / 5 × 1.10 = 1.98 accidents per year
Typical accident rate AT
AT = (b0 × Sadj) × X (from appendix A6.5, rural two lane roads)
where: coefficient b0 = 16 from table A6.12(a), for a mean seal width
of 8.2 m, for 1,000 to 4,000 AADT on level terrain
Exposure (X) = 3.3 km × 2,800 AADT × 365 / 108 = 0.034
Sadj = 1.21 from table A6.13. This adjusts b0 upward, because
the current seal width of 7 m is narrower than the mean seal
width of 8.2 m for a road that carries 1,000 to 4,000 vehicles per
day.
AT,dm = 16 × 0.034 × 1.21 = 0.66 accidents per year.
No adjustment is required for time zero as year zero is 2006.
Weighted accident rate AW for the do minimum
The weighted accident rate equation from appendix A6.3 is:
AW,dm = w × AT + (1 - w) ×AS
w =
The accident history at this site is considered reliable and so is the typical accident rate, therefore aX and aM are both equal to one, and the equation for w reduces to:
w =
with k = 0.8 (from table A6.12(b))
Because k is per kilometre, AT needs to be divided by the site length (3.3 km), therefore AT = 0.66 / 3.3 = 0.200
w =
= 0.80
Therefore the weighted accident rate is:
AW,dm = 0.80 × 0.66 + (1 − 0.80) × 1.98
= 0.92 accidents per year
Do minimum accident costs
= 0.92 × $555,000 (from table A6.22)
= $510,600 per year
Option (a) accident costs: no significant changes at site
Typical accident rate AT
AT,opt = b0 × exposure × cross − section adjustment factor
= 16 × 0.034 × 0.69 = 0.38 accidents per year
where: the cross−section adjustment factor from table A6.13 adjusts b0
downwards as the proposed seal width of 9 m is wider than the mean
seal width of 8.2 m (for a road with 1,000 to 4,000 vehicles per day).
Weighted accident rate AW for the option
AW,opt = AT,opt × AW,dm / AT,dm (from appendix A6.3)
= 0.38 × 0.92 / 0.66 = 0.53 accidents per year
Option (a) accident costs
= 0.53 × $555,000
= $294,150 per year
Option (a) accident benefits
= $510,600 − $294,150
= $216,450 per year
Option (b) accident costs: site significantly changed
If the proposed improvement is considered a fundamental change, in this case due to other works such as the protection of steep drop−offs or removal of obstacles in the roadside clear zone, then the site specific accident history used in the weighted accident procedure (Method C) is not relevant in the calculation of the option accident rate and costs. When there is a fundamental change the accident costs for the option are calculated using Method B.
Typical accident rate AT for option
AT,opt = b0 × exposure × cross−section adjustment factor
= 16 × 0.034 × 0.69 = 0.38 accidents per year
Option (b) accident costs
= 0.38 × $555,000
= $210,900 per year
Option (b) accident benefits
= $510,600 × $210,900
= $299,700 per year.
Option (b) accident costs: site significantly changed
If the proposed improvement is considered a fundamental change, in this case due to other works such as the protection of steep drop−offs or removal of obstacles in the roadside clear zone, then the site specific accident history used in the weighted accident procedure (Method C) is not relevant in the calculation of the option accident rate and costs. When there is a fundamental change the accident costs for the option are calculated using Method B.
Typical accident rate AT for option
AT,opt = b0 × exposure × cross−section adjustment factor
= 16 × 0.034 × 0.69 = 0.38 accidents per year
Option (b) accident costs
= 0.38 × $555,000
= $210,900 per year
Option (b) accident benefits
= $510,600 × $210,900
= $299,700 per year.
