Appendix B.5
Contents
Summary ...189
I Introduction ...191
II Description of the Northern Expressway project ...193
III Review of ex-ante CBAs ...195
Ex-ante CBAs ...195
Review of DTEI 2007 CBA study ...196
IV Accuracy of ex-ante traffic forecasts and CBA ...201
Parsons Brinckerhoff (2011) study ...201
Accuracy of ex-ante traffic forecasts ...207
Accuracy of ex-ante travel time saving predictions ...211
V Lessons learnt ...217
CBA reporting ...217
Traffic analysis ...217
Travel time modelling ...218
References (additional) ...219
List of tables
Table 1 Ex-ante CBAs for the proposed Northern Expressway project ...195
Table 2 Traffic forecasts for the base case and project case ...197
Table 3 Travel time savings for the Northern Expressway project (minutes) ...206
Table 4 Ex-ante traffic forecasts for the base case (vehicles per day, 2006–2011) ...207
Table 5 Growth of annual average daily traffic on the Northern Expressway (per cent per year) ...208
Table 6 Travel time savings for the Northern Expressway project (2010–11) ...211
Table 7 Assumptions about future traffic growth and congestion ...214
List of figures
Figure 1 Proposed Northern Expressway project ...194
Figure 2 Annual travel time cost savings (in 2006 prices) ...198
Figure 3 Growth in annual travel time savings (% change over the previous year)...198
Figure 4 Annual VOC savings (in 2006 prices) ...199
Figure 5 Travel time survey routes ...203
Figure 6 Traffic impacts of Northern Expressway ...205
Figure 7 Forecast traffic volumes over actual observations (%, 2011) ...208
Figure 8 Actual and forecast traffic volumes on the Northern Expressway ...209
Figure 9 Travel time cost savings: ex-post versus ex-ante ($m) ...215
Summary
The ex-post evaluation of the Northern Expressway project forms part of the second round of case studies undertaken by the Bureau of Infrastructure, Transport and Regional Economics (BITRE) of road investment projects on the National Land Transport Network. This case study was undertaken in consultation with SA Department of Planning, Transport and Infrastructure (DPTI). Objectives were to assess the economic performance of the Northern Expressway project and to check the accuracy of the ex-ante cost-benefit analysis (CBA).
The project was to construct the new 23 kilometre-long Northern Expressway and upgrade 15 kilometres of the existing Port Wakefield Road. The project was completed in 2010 within the budget of $564 million (outturn).
Unlike other studies which involved full ex-post evaluation, this case study focused largely on reviewing the ex-ante CBA and checking the accuracy of ex-ante travel time saving estimates using readily available information. A number of lessons can be learnt:
CBA reporting: a series of CBAs were performed for the project, but the differences were not discussed in the final CBA for decision making. The significant fluctuation in travel time saving estimates contained in the scoping and development Project Proposal Report (PPR) should have been clearly explained. Doing this would have helped reduce errors and improved decision making.
CBA documentation for the project comprised the PPR and printout of CBA spreadsheets as one of the PPR’s appendices. For a project worth over half a billion dollars, a more detailed self-contained CBA report should have been requested to facilitate review and decision making.
The ex-ante CBA report should have provided a critical review of traffic modelling results before they were used as inputs for the CBA. This is especially true for those inputs directly impacting travel time cost savings. Such a review could include:
• a summary of modelled (or assumed) traffic growth forecasts for both the base and project cases (see tables 4 and 5 as an example)
• a graph of modelled growth in travel time savings over time (see figure 3)
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BITRE • Report 145 Volume 2 Case Studies
congestion impacts. The reasons for this could not be identified without undertaking a detailed ex-post traffic modelling within the MASTEM. Valuable lessons could have been learnt if such modelling had been undertaken.
Travel time modelling: Travel time savings were over-estimated due to overly optimistic traffic forecasts. It was not clear whether the modelling methodology itself was also a contributor factor.
The simplified methodology developed in this case study, while rough, can be useful to crosscheck results derived from complex traffic models. It can also provide faster analyses with minimum data requirements. However, this should only be used as a last resort in the absence of any other alternatives.
I Introduction
The Northern Expressway case study forms part of BITRE’s second round of ex-post cost-benefit analysis (CBA) of road investment projects on the National Land Transport Network. The case study was implemented by BITRE in consultation with the SA Department of Planning, Transport and Infrastructure (DPTI, formerly known as DTEI).
The objectives are to:
• assess the economic performance of the project
• check the accuracy of ex-ante CBA
• explain differences (if any) in results between the ex-ante and ex-post CBAs
• draw lessons from the case study to improve future CBAs.
As part of the ex-post economic evaluation, this case study provided an opportunity to review the performance of an urban transport model to predict the base case travel demand and assess the impact of a major piece of infrastructure investment in an urban environment. Data on travel times on key existing roads and the newly-built expressway were used to scrutinise travel time saving estimates from the Metropolitan Adelaide Strategic Transport Evaluation Model (MASTEM), an urban transport planning model for Adelaide.
The following section provides an overview of the Northern Expressway project. Section 3 reviews the ex-ante CBA analyses undertaken by Department for Transport, Energy and Infrastructure (DTEI, now known as DPTI). Section 4 checks the accuracy of ex-ante traffic forecasts and travel time saving estimates using actual observed information and more recent traffic forecasts. Lessons learnt are discussed in the final section.
