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Members and Friends of IFRTT Representatives from the local trucking industry have recently requested Queensland Transport to revisit its position on increasing the mass limit for two axle trucks with a rear dual-tyred single axle. The request is based upon a discrepancy in both domestic and international mass limits. Queensland currently allows a limit of 9 tonnes, while Victoria allows 10 tonnes for vehicles fitted with road friendly suspensions. European Union limits, while adhering to Directive 96/53/EC, are higher again at up to 11.5t. While practically all of Queensland's pavements are composed of an unbound granular pavement with a thin bituminous seal, the principle of pavement degradation neutrality under RFS is maintained. Therefore dynamic performance is viewed as the most prevalent issue, with static roll threshold (SRT) being of the most concern. Two reports have been previously commissioned by the National Transport Commission (www.ntc.gov.au) in assessing the safety performance of these vehicles at higher mass (Sweatman, 1999 and Pearson/Prem, 2002). Computer simulations showed that vehicle configurations which utilise dual-tyred single axles can have relatively low levels of stability performance under current single axle mass limits, especially in the case of two-axle rigid trucks. The key finding of the computer simulations was that air-suspended axles at 10.0 tonnes provide stability significantly better than that provided by mechanically-suspended axles at 9.0 tonnes. This suggests that, at face value, refusing an increase would be difficult to justify. Please note that it was on the basis of these findings that Victoria allowed the increase. Queensland and other Australian states have maintained a 9 tonne limit. While finalisation of Australian Performance-Based Standards for infrastructure is relevant, Queensland has primarily maintained the 9 tonne limit on safety grounds. There are two major reasons for this. Firstly, there is no guarantee that RFS roll stiffness would be maintained to the levels assumed in computer simulation. Secondly the SRT values for both 9.0 tonnes/mechanical suspension and 10.0 tonnes/air suspension are well below Australia's performance-based standard of 0.35g (even though simulation showed an increase in SRT on air suspension). In summary, the research appears to suggest that these vehicles are poor dynamic performers. However, if this in fact the case, the road freight industry is asking why European Union member states allow limits of up to 11.5t. Therefore, as part of reviewing literature in preparing a response to industry's request, we are looking at the following aspects: 1. Are these vehicles over-represented in rollover accidents? Ideally, there would be some statistical correlation. 2. Are there more stringent driver competency requirements in the EU (or other areas with higher limits such as Latin America) to compensate for increased mass? 3. Are there any vehicle specific dimensional limits (constructed or loaded height, width and so on) in place to improve the roll stability performance of these vehicles? We have already contacted and gratefully received further advice from Peter Sweatman and Bob Pearson. We would very much appreciate any advice on any other studies that you may know of, or any other aspects we should be covering. Regards, Les Les Bruzsa (Principal Engineer) Strategic Policy Team Land Transport and Safety Division Queensland Transport PH (617) 3253 4205 Fax (617) 3253 4211 ************************************************************ Opinions contained in this e-mail do not necessarily reflect the opinions of the Queensland Department of Main Roads, Queensland Transport or Maritime Safety Queensland, or endorsed organisations utilising the same infrastructure. If you have received this electronic mail message in error, please immediately notify the sender and delete the message from your computer. ************************************************************