A range of speed-flow curve relationships are currently in use by road authorities across jurisdictions in Australia. These speed-flow relationships are used by the authorities in carrying out traffic modelling but also feature in road project evaluations of mostly urban and, to a lesser degree, rural road projects. The way some of these relationships are specified becomes critical when volume capacity ratios (VCR) approach and exceed a value of 1.0 because different queuing speeds are assumed at different VCR levels. This study examines the implications for project appraisal of using different queuing speeds at various VCR levels. The study first reviews speed-flow curves used internationally and across jurisdictions in Australia, e.g. the Akcelik model and Bureau of Public Roads (BPR) curve. Using examples drawn from a road authority and the Austroads Guide to Project Evaluation, it then investigates the impact on project evaluation results of using different queuing and saturation speeds at different VCR levels. The study finds that using different queuing speeds at various VCR levels affects evaluation results (e.g. Net Profit Value [NPV] and Benefit Cost Ratio [BCR]). The report also documents the responses from Australian jurisdictions regarding the harmonisation of speed-flow relationships (queuing speeds and VCR levels) across jurisdictions. The study finds that road authorities are not in favour of harmonising speed-flow relationships per se because of differences in local road networks and traffic conditions reflected in their models. However, they do acknowledge the importance and impact of the VCR-queuing speed relationship as significant in terms of its impact on economic appraisal results and the comparison of those results across jurisdictions. A subsequent decision by the Austroads Evaluation and Planning Technical Review Panel was that a default queuing speed of 30 km/h be adopted at VCR = 1 where applicable, instead of 8 km/h suggested by the traditional NIMPAC style models underpinning most of the rural network models employed in Australia. The 30 km/h queuing speed estimate is consistent with that used in HDM-4 and also reflects queuing speeds reported in the Austroads Guide to Traffic Management. (Author/publisher)
Samenvatting