Methods for Quantitative Analysis

There are six methods that may be used to undertake a quantitative assessment of the risk

associated with a hazard and the appropriate treatment for a hazard. Jurisdictions in Australia and New Zealand may use one or several of these methods depending on the complexity of the

hazards alongside the section of road that is being investigated.

Similar basic principles are applied in all the methods in that they have the objective of minimising the number of crashes and the severity of crashes. Most methods involve an economic analysis.

However, the methods vary in the extent of analysis and the details of the analysis. A related product is the Road Safety Risk Manager, a product developed by ARRB that is described in Commentary 7.

The methods available are:

ƒ a simple manual method

ƒ a detailed quantitative manual method

ƒ the Roadside Incident Severity Calculator (RISC program) developed by the Queensland Department of Transport and Main Roads

ƒ the Roadside Safety Analysis Program (RSAP) that is the current USA method developed through the National Cooperative Highway Research Program (AASHTO 2003)

ƒ a method developed by RTA that calculates the risk associated with a hazard and compares it to an intervention benchmark for the particular type of road.

4.5.5 Simple Manual Method

In circumstances where a hazard exists beside the road a simple procedure may suffice to determine whether the hazard requires treatment. The method involves:

ƒ An assessment of the likelihood that a vehicle will crash into an object or feature and the severity of the crashes.

ƒ An assessment of the number of crashes that would occur with a treatment and the severity of those crashes.

ƒ A comparison of the number and severity of crashes that would occur at an untreated hazard with the number and severity of crashes if a treatment was provided (e.g. road safety

barrier).

The process shown in Figure 4.8 involves:

ƒ An assessment as to whether the object or feature has high severity attributes requiring consideration of the severity of the object (refer to Section 4.6.3 and relevant tables in Appendix E).

Does the

Source: Based on QDMR (2005).

Figure 4.8: Simple process for risk assessment 4.5.6 Detailed Manual Method

The detailed manual method is based on a risk assessment and economic analysis as discussed in Section 4.5.2 and Section 4.6.

The process involves:

ƒ From crash data for an existing situation or from the process in Section 4.6, determine the annual crash cost for the hazard.

ƒ Using the guidance in Section 5 and Section 6 develop options for treatment of the hazard.

ƒ Estimate the costs associated with each treatment option including crash costs (e.g. crashes into a road safety barrier), construction costs, annual maintenance costs and operating costs where applicable. Note that the process in Section 4.6 can be used to establish crash costs associated with treatments that have some risk for road users. An example of the calculation of crash costs associated with a hazard compared to a road safety barrier is provided in Appendix F.

ƒ A benefit-cost analysis of the whole of life costs should then be undertaken of the existing situation (i.e. the untreated hazard) and of all treatment options identified for evaluation and consideration. This process is described in Section 4.5.1 and Section 4.5.2. Refer also to the Guide to Project Evaluation – Part 2: Project Evaluation Methodology (Austroads 2005).

ƒ Use the results of the benefit-cost analysis in conjunction with any qualitative analysis (Section 4.5.3) to establish which treatment should be recommended.

RISC requires the user to model roadside objects and potential treatments for these objects using an array of numerical parameters. Once this is done the relative benefits and costs for different treatments are automatically calculated using an algorithm based on the AASHTO Roadside Design Guide (AASHTO 1996). The most cost-effective treatment for each hazard can be determined and the decision making process can then continue to the next step. The program operates through a series of windows and menus.

The modelling can be used to determine the possible benefit-cost ratios achievable by comparing the treatment options available. For example, a comparison can be made between leaving an end-on culvert as it is, installing bar grates, redesigning the end wall to reduce its severity, and the installation of a road safety barrier.

The method and processes adopted by the RISC software for determining the impact frequency of errant vehicles and calculating crash costs is outlined in Appendix G.

4.5.8 Roadside Safety Analysis Program

The Roadside Safety Analysis Program (RSAP) software is described in Appendix A of the Roadside Design Guide (AASHTO 2006; Amendment Chapter 6 – median barriers 2006). The information in the appendix advises that RSAP was developed under the National Cooperative Highway Research Program Project 22-9 (AASHTO 2003) and represents one approach to using the Roadside Design Guide. In addition, in reference to RSAP it is stated that:

It carries no guarantees or warranties from the American Association of State Highway and Transportation Officials. The RSAP program is intended as a tool for economic analysis and should not supersede the guidelines presented in the Roadside Design Guide or sound engineering judgement.

This condition seems to correctly suggest that RSAP is a tool and practitioners who use it should understand how it operates and be responsible for the data used and the results it generates. Like all software, used appropriately RSAP is considered to be a useful tool that could be utilised by Australian and New Zealand jurisdictions.

RSAP was developed because deficiencies were identified in previous AASHTO software entitled ROADSIDE, and as a result of the Federal Highway Administration (FHWA) adopting the NCHRP Report 350, Recommended Procedures for the Safety Performance Evaluation of Highway

Features (Ross et al. 1993), as the official guidelines for safety performance evaluation of highway features. As a result, RSAP provides an improved computer-based cost-effectiveness analysis procedure for use in:

ƒ assessing alternative roadside safety treatments at both point locations and for sections of roadway

ƒ developing warrants and guidelines including those which consider performance levels of safety features.

RSAP provides a simple and structured means for data entry and four separate reports summarising the analysis results and input data, namely:

ƒ Benefit-cost ratio – Presents the incremental benefit-cost (b/c) ratios associated with the

ƒ Alternative cost – Presents the predicted crash frequencies, and the annual installation, maintenance, and repair costs associated with each of the alternatives in a tabular format.

ƒ Feature cost – Presents the predicted impact frequencies, average severity, and crash costs associated with individual features of each alternative in a tabular format.

ƒ Input data – Presents the input data for each alternative in a summary form.

4.5.9 Alternative Crash Risk Method

An alternative crash risk method that may be considered generally follows the same process as the hazard risk assessment that is shown in Figure 4.10 except that a different approach is taken beyond Step R1g as shown in Figure 4.9, namely:

Step R2 – Determine the crash consequences

The crash consequences (another term for severity) are determined from a suite of tables similar to those described in Section 4.6.3 and presented in Appendix E that provide crash consequence indices for various roadside features and objects.

Step R3 – Determine the probability of injury and fatality

This step involves the use of a table that relates the crash consequence indices to the probabilities of injury or fatality. The values from the table are used as an input to Risk Step R4.

Step

R1g Calculate the frequency of errant vehicle crashes

Determine the crash consequences

Determine the probability of injury and fatality

Determine the ‘fatality-equivalent’ risk R4

R2

R3 Step

R1g Calculate the frequency of errant vehicle crashes

Determine the crash consequences

Determine the probability of injury and fatality

Determine the ‘fatality-equivalent’ risk R4

R2

R3

Note: This process is an alternative approach beyond Step R1g in Figure 4.10.

Source: Adapted from RTA (2008).

Figure 4.9: Part of the alternative crash risk assessment process

Step R4 – Determine the risk

The risk of injury and the risk of fatality are calculated separately. The risk of injury is divided by a factor of 14 to reflect the difference in injury crash costs and fatality crash costs. This allows for the lesser magnitude of injury consequences to be compared with those leading to acute injury or fatality. The risk of fatality per crash is multiplied by the number of crashes per year and the combined risk level of the hazard or treatment option is calculated as follows:

Risk (combined) = crashes per year (N) x [risk _(injury) / 14 + risk _(fatality)]

The combined risk is then compared to a benchmark for the type of road to determine the required action. If the combined risk is below the benchmark, consideration is given to the consequences of leaving the hazard as it is, and if it is greater than or equal to the benchmark treatment options are considered. Designers should refer to RTA (2008) and the Roads and Traffic Authority NSW for further information.