7.4 - Accident reduction benefits

7.4 - Accident reduction benefits

Introduction

A transport service proposal may reduce accidents by moving passengers or freight to safer modes of transport, such as buses and rail. While this may be an outcome of a transport service proposal, it is seldom the primary objective.

Accident occurrence (and accident cost) is affected by:

  • trip diversion
  • changes in travel demand
  • a reduction in the number of potential conflicts between different modes.

Nature of accident benefits

Trip diversion from road to rail or bus will generally provide positive benefits to users that change mode. The accident risk (likelihood of having an accident) and accident costs of remaining users will be similar.

A reduction in the number of potential conflicts between modes will generally lead to positive benefits, by reducing the number of conflicts and in many cases the accident severity. Accidents between bus/rail and private motor-vehicles tend to be more severe than those between two private motor-vehicles.

Accident evaluation procedures

There are three accident analysis methods used by Land Transport NZ; accident-by-accident analysis, accident rate analysis and the weighted accident procedure (refer to appendix A6 of volume 1). In transport service proposals analysts should use accident rate analysis.

Accident rate analysis makes use of predictions of the reported injury accident rate from areas that are similar to the proposed transport service location. For a transport service proposal such as a rail service, accident rates for both road and rail must be used to predict the number of accidents and the subsequent costs. Roads should also be separated into urban and rural sections.

Volume 1 worksheets A6.7 (a) and (b), and A6.8 (a) and (b) can be used for analysing urban and rural road routes respectively.

Accident rates and prediction models

Accident prediction models and accident rate equations are not provided for rail, buses or coastal shipping. Analysts are permitted to use accident prediction models and accident rate equations from other sources, as long as the robustness of these other sources can be demonstrated.

Urban transport services - accident rates

The accident prediction models in table A6.3(a) of appendix A6 (volume 1) can be used to calculate accident rates for urban roads. The models predict accidents between major intersections (or on links). An adjustment factor of two may be used to estimate the total number or reported injury accidents on both the links and at intersections for urban roads with intersections when the frequency of intersections along a road and the volume of crossing traffic is fairly typical. This is based on an assumption that approximately 50 percent of accidents occur at intersections.

On some urban roads, particularly in the middle of towns and cities, intersections are often closely spaced and this factor is not valid. When either of these two factors is atypical, then analysts should use the intersection prediction model in A6.5.3 of appendix A6 (volume 1) to calculate accident rates at the intersections. If the proportion of the trip on atypical roads is short then this issue can be ignored. A validated transportation model can be used to assist in more complex situations.

Rural transport services - accident rates

For freight transport service proposals, where the road network affected by the proposal is primarily rural in location, accident rate equations for heavy vehicles only are used to estimate the reduction in freight related accidents. This is a subset of the accidents given by the equation in A6.5.6 of appendix A6 (volume 1).

Heavy vehicle reported injury accidents/year   =   b0X

Where X is the exposure in 100 million vehicle kilometres, and the coefficient b0 is given in the following table.

Rural mid-block equation coefficients (b0) for heavy vehicle accidents:

AADT Coefficients b0 by terrain type
Level(0 to 3%) Rolling(3 to 6%) Mountainous (>6%)
<= 4,000 20 42 52
> 4,000 20 20 43

Each freight route should be broken down by traffic volume and terrain type. The terrain type can be selected by analysing the route gradient data. The gradient bands for each terrain type should generally be maintained throughout each section. Sections of road that are less steep can occur in rolling or mountainous sections for short lengths. This is allowed provided that the lower gradient length is followed by another rolling or mountainous gradient. The appropriate accident rate is then used for each section.

Procedure for accident rate analysis

For each mode that will be affected by the transport service proposal, calculate the accident reduction benefits as follows:

Step Action
1 If the case to be assessed consists of predominately radial then arterial, collector and local routes, with a standard density of intersections, and motorways, or rural roads then use this procedure (for each option).

If the case to be assessed consists of a complex road network or arterial routes with very high or low density of intersections then use output from a validated transport model in conjunction with accident prediction models from appendix A6 (volume 1).
2 Where the transport service proposal affects urban road(s):
  • Using worksheet A6.7, record for each mid-block road type (and land use) the length, current AADT, and the predicted AADT after implementation of the transport service proposal. Where there is more than one length of any given mid-block road type and the AADT varies, an average of the AADT values can be used (alternative add rows to the bottom of worksheet A6.7).
  • Using the coefficients (b0 and b1) provided, calculate the do minimum accident rate (Adm) for each mid-block road (link) type using the current AADT: A = b0 × QT b1 × L
  • Calculate the accident rate (Aopt) for each of the options, using the above accident rate equation and the AADT after implementation.
  • Intersection adjustment for collector and arterial road links only. Multiply the accident rates (Adm and Aopt) for the appropriate road links by 2 to derive the adjusted accident rates.
3 Where the transport service proposal affects rural road(s):
  • Using worksheet A6.8, record for each section of road the length, AADT, terrain type and the daily number of heavy commercial vehicles that currently use the route and the daily number of heavy vehicle trips that will use the route following implementation of the transport service proposal.
  • Calculate the HCV exposure (in 100 million vehicle km per year) for both the do minimum and each option using the current and option number of truck trips.
  • Multiply the HCVk exposure by the appropriate co-efficient (b0) to determine the do minimum and option reported injury accidents per year (Adm and Aopt).
  • Using worksheet A6.8 record for each section of road the length, AADT, terrain type and the daily number of heavy commercial vehicles that currently use the route and the daily number of heavy vehicle trips that will use the route following implementation of the transport service proposal.
  • Calculate the HCV exposure (in 100 million vehicle km per year) for both the do minimum and each option using the current and option number of truck trips.
  • Multiply the HCVk exposure by the appropriate co-efficient (b0) to determine the do minimum and option reported injury accidents per year (Adm and Aopt).
4 For other modes (ie existing transport services):
  • Develop or obtain accident rates for other modes (ie rail and buses). The accident rate will be based on a factor, such as the total number of passengers, annual tonnage, or similar. The factor selected will depend on the information available about accidents for the mode under consideration.
  • Calculate the change in the factor being used based on the projected demand for trips on other modes after implementing the transport service (from chapter 4).
  • Calculate the do minimum and option number of accidents resulting from the transport service proposal for each 'other' mode for the current value of the factor (eg change in number of kilometres travelled, change in number of passengers, etc) and the increased value of the factor.
5
  • Multiply the do minimum and option number of accidents for urban and rural roads and 'other' modes by the appropriate standard accident costs - 'all other sites' costs in table A6.13 in appendix A6 (volume 1) for roads.
  • Calculate the accident benefits for each option affected by the implementation of the transport service by subtracting the option accident costs from the do minimum accident costs for rural and urban roads and on other modes.
  • Sum the accident benefits and disbenefits on urban and rural roads and on 'other' modes.
  • Enter the total accident reduction benefit for each option into the reporting table at the end of this chapter.