A pro-forma approach to car-carrier design
To mitigate accidents, reduce loss of life and to protect road infrastructure, it is important that heavy vehicles are regulated. Regulatory frameworks can be divided into two main groups: prescriptive and non-prescriptive. The prescriptive regulatory framework is currently the norm in South Africa and the majority of countries worldwide. Road safety in this framework is governed by placing constraints on vehicle mass and dimensions. These parameters can be measured by the law enforcer and if these are found to exceed prescribed limits, the vehicle is deemed unfit for road use. Although such a legislative framework is simple to enforce and manage, prescriptive standards inherently impose constraints on innovative design and productivity, without guaranteeing vehicle safety. An alternative regulatory framework is the performance-based standards (PBS) framework. This alternative non-prescriptive framework provides more freedom and directly (as opposed to indirectly) regulates road safety. Limits regarding overall length and gross combination mass (GCM) are relaxed but other safety-ensuring standards are required to be met. These standards specify the safety performance required from the operation of a vehicle on a network rather than prescribing how the specified level of performance is to be achieved. On 10 March 2014, the final version of the South African roadmap for car-carriers was accepted by the Abnormal Loads Technical Committee. The roadmap specified that, from thereon, all carcarriers registered after 1 April 2013 would only be granted overall length and height exemptions (which logistics operators have insisted are essential to remain in business) if the design is shown to meet level 1 PBS performance requirements. This resulted in an increased demand for car-carrier PBS assessments. One significant drawback of the PBS approach is the time and expertise required for conducting PBS assessments. In this work a pro-forma approach is developed for assessing future car-carrier designs in terms of their compliance with the South African PBS pilot project requirements. First, the low-speed PBS were considered and a low-speed pro-forma design was developed by empirically deriving equations for frontal swing, tail swing and low speed swept path. These were incorporated into a simplified tool for assessing the low-speed PBS compliance of car-carriers using a top-view drawing of the design. Hereafter, the remaining PBS were considered, incorporating additional checks to be performed when evaluating a potential vehicle. It was found necessary to specify a minimum drive axle load in order to meet the startability, gradeability and acceleration capability standards. The required drive axle load was determined as 19.3% of the GCM. It was confirmed that the static rollover threshold performance can accurately be predicted by means of the applicable New Zealand Land Transport Rule method. The study is limited to 50/50-type car-carriers, however the methodology developed will be used to construct assessment frameworks for short-long and tractor-and-semitrailer combinations. The pro-forma approach offers a cost-effective and simplified alternative to conventional TruckSim® PBS assessments. This simplified approach can significantly benefit the PBS pilot project by offering a sustainable way to investigate the PBS conformance of proposed car-carriers.