A pro-forma approach to car-carrier design
Date
2016
Authors
Benade, Richardt
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Abstract
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 TruckSimĀ® 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.