A6.4 - Accident trends

Introduction

This section provides guidance on the adjustment of accident numbers for general accident trends.

General accident trends

Since 1985, there has been a downward trend in reported traffic accidents. At the same time that accident numbers have decreased, traffic volumes have increased, indicating that accidents rates have decreased more than accident numbers.

The combination of these two factors means that typical accident rates established from past research and site specific accident numbers need to be adjusted in order to give a realistic estimate of the likely accident situation at the project site in the future.

The adjustment to accident numbers is a two stage procedure, with the first adjustment being to modify the accident numbers at time zero and the second adjustment being to modify the growth rate used for discounting accident benefits to take account of the forecast continued trend after time zero.

There have been marked differences between the accident trends in 50 km/h areas compared with 70 km/h and above areas, and different factors are used to modify the accident numbers for the different posted speed limit areas.

Table A6.1(a) provides factors to convert historic average accident numbers to time zero for Method A. For Method B, an equation is provided to adjust the rate to time zero.

Table A6.1(b) provides factors to modify the predicted future traffic growth rate when discounting the accident cost savings.

Adjustment to time zero

Accident numbers and rates for project evaluation are to be determined for time zero. This requires adjusting the observed or predicted number of accidents assessed at the mid-point of the analysis period to time zero. The procedure differs if using accident history (accident-by-accident analysis) or accident prediction models or rates (accident rate analysis).

Method A adjustment

This procedure should be followed if using accident-by-accident analysis. From table A6.1(a), select the appropriate adjustment factor for the site based on its traffic growth rate and posted speed limit. For example, for a project where the posted speed limit is 50 km/h and the traffic growth rate is 2 percent at time zero, the accident numbers will be factored by 0.90 to adjust the accident numbers to time zero.

Method B adjustment

This procedure should be followed if using accident rate analysis. As the prediction models and equations in appendix A6.5 use historical accident data, the predicted number of accidents needs to be adjusted for accident trends.

A = A_T × (1 + f_t (y_z − 2006))

where:

A is the accident rate adjusted for accident trends

A_T is the typical rate found from models or rates

f_t is the factor for adjusting the typical rate

• −0.03 for sites with speed limits 60km/h and below
• −0.01 for sites with speed limits 70 km/h and above

y_z is year zero of the analysis period

Table A6.1(a) Accident trend adjustment factors

Adjusting traffic growth rate for discounting

When discounting the accident cost savings from time zero forwards the predicted growth rate is adjusted to reflect the predicted continued trend in accidents. Table A6.1(b) provides the adjustments to use for the different speed limit areas.

Using the factors in table A6.1(b) it is possible for the accident growth rate used for discounting to be negative if the predicted traffic growth rate at the site is less than 3 percent in 50 km/h areas or 1 percent in 70 km/h and above areas. For example, if the site is in a 50 km/h posted speed area and the traffic growth rate for the site is 1.5 percent then the growth rate to use for discounting accident costs is 1.5 - 3 = −1.5, ie, −1.5 percent is entered in the discounting equation.

Table A6.1(b) Growth rate adjustment factors