On behalf of AUSTROADS, a heavy vehicle swept path specification study has been undertaken by the Australian Road Research Board and Road User Research Pty Ltd. The overall aim of the study was to develop appropriate criteria of acceptable low-speed geometric performance and a practical means of verifying compliance. The study encompassed all classes of heavy vehicles: general freight vehicles, buses, medium combination vehicles and road trains. A review of the literature showed that, while the theory of offtracking of articulated vehicles is reasonably complex, some relatively simple parameters are available to rate vehicles for offtracking performance. Where detailed analysis is required, the VEHICLE/PATH program developed by the Queensland Main Roads Department and ARRB, offers excellent technical capabilities, and is highly recommended. A review of current Australian practice found that current regulatory controls on swept path are indirect and rely on vehicle dimensional limits introduced by the 1975 NAASRA Economics of Road Vehicle Limits Study and modified in 1985 by the NAASRA Review of Road Vehicle Limits. Reasons for controlling swept path are generally related to safety and road space requirements and there is a strong desire to prevent continual increases in swept path requirements as vehicle dimensions are relaxed from time to time. Road designers have some difficulties with defining design vehicles and steering paths for intersections, and there are some concerns with the level of enforcement of dimensional limits. At the same time there is considerable industry pressure for longer, more productive semi-trailers up to 13.7 m in length and anecdotal evidence that such vehicles already exist on the road system. Field testing using the ARRB trailer provided data on steering paths, clearances to road features, vehicle speed and turning parameters. Analysis of this data provided (i) critical swept path situations for each vehicle class (ie, steering path geometry plus the critical swept path dimension), (ii) vehicle-based criteria of swept path performance and (iii) means of assessment against these criteria. It was found that truck drivers steer in such a way as to control clearances to the rear axles, and that the total swept path at the exit from the turn is usually the most critical dimension. Taking into account implications for dynamic stability and load distribution characteristics of vehicles, various means of assessing swept path performance against the specified criteria were developed. These cover the simplest measures (required for enforcement in the field) through slightly more complex parameters (suitable for use at registration and Australian Design Rule approvals) to computer simulation for one-off use. Detailed recommendations for swept path assessment of all vehicle classes are included in the report. The capacity of the road network needs to be established in order to set a road-based swept path performance specification.
Abstract