EEM1 (road infrastructure) Appendix 6 - Accident costs

Introduction

This appendix gives guidance on calculating accident cost savings for the do minimum and project options.

For the purposes of this manual, an accident is an event involving one or more road vehicles that results in personal physical injury and/or damage to property.

When to do an accident analysis

Not all project evaluations require an accident analysis. An accident analysis may be appropriate where one or more of the following is true:

(a) at intersections or sites less than 1 km in length, within the last 5 years there have been:

• 5 or more injury accidents; and/or
• 2 or more serious or fatal accidents;

(b) at sites longer than 1 km in length, within the last 5 years there have been:

• 3 or more injury accidents per km; and/or
• 1 or more serious or fatal accident per km;

(c) there is some commonality amongst the accidents that have occurred;

(d) a recognised accident investigation specialist considers that the site has significant safety deficiencies (eg, high accident risk sites);

(e) there is a high level of public concern;

(f) there will be a fundamental change in the site where the types of accidents or level of accident severity will change significantly, such as:

• improvements will significantly alter the traffic speed or the distribution of traffic within a network; or
• new links or intersections are being added to a road network.

Low volume roads

For sites with AADT less than 1,500 vehicles per day (vpd) that do not meet requirement (a) or (b) above, the last ten year history can be used. The ten year history must be divided by two to obtain an equivalent five year history for analysis.

Remote and near rural roads

Remote rural roads are sites carrying less than 1,000 vpd and are more than 20 km away from a town with a population of 3,000 or more. Other rural sites are near rural.

Accident analysis methods

There are three accident analysis methods available:

• method A: Accident by accident analysis
• method B: Accident rate analysis
• method C: Weighted accident procedure.

Appendix A6.2 gives guidance as when to undertake an analysis and what method(s) to use.

General process

The general process for an accident analysis is as follows:

(a) Select the appropriate analysis procedure(s) using appendix A6.2 and, depending on the method(s) selected:

• determine the historic accident performance by analysis of accident records, typically over the last 5 years;
• select the accident prediction models or exposure-based accident prediction equations for the do minimum and project options from appendix A6.5.

(b) Assess the annual accident performance and corresponding accident costs for the do minimum and the project options. Adjust for general trends in accident occurrence.

(c) Calculate the annual accident cost savings. These are the future annual accident costs of the do minimum less the future annual accident costs of the project options.