Weight and dimension characteristics of large truck combinations as a function of regulatory limits : Manitoba highways

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Plett, Randy
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Examination of gross vehicle weight and axle weight distribution patterns of various vehicle types on different Manitoba highways in different years indicated a substantial degree of repetition which suggested a good possibility of developing models of these loading characteristics based on the regulatory environment within which the trucks have operated. Data from the Manitoba Department of Highways and Transportation Truck Weight and Dimension surveys (1972-1986) was used in determining these weight distributions under differing sets of weight regulations. Models were then formulated by fitting mathematical formulae to these weight distribution curves. The models showed, with one notable exception, a general pattern of increased vehicle weight characteristics corresponding to increase in regulatory limits. This confirms the hypothesis that governing weight limits do have a demonstrable effect on how trucks are loaded and thus on the actual weights observed in the field. Furthermore, it was shown that derived characteristics such as equivalent single axle loads and payload distributions were also dependant, in a large part, on these same regulatory limits and hence could be estimated through the use of the previously mentioned models. A secondary objective of this research was to develop a standardized summary of the Manitoba truck survey results in a form which facilitates its use by the transportation planning, engineering and research communities. This was done by collecting all data from 1972 to 1986 and placing them on one computer accessible magnetic tape in a format which eliminates inconsistencies in the surveY format across the time base. Another objective was to make use of the survey data in order to provide insight into changes in the large truck fleet mix operating in Manitoba since the 1970's and to investigate changes in the physical characteristics of this fleet. In this vein, it was discovered that: (i) the size of the truck fleet in Manitoba has been increasing, (ii) the average size of the vehicles which make up this fleet has been increasing, (iii) both the power/weight ratio and the turning performance of many vehicles are less than those of design vehicles, (iv) typical tandem axle spreads are consistently larger than those used in the AASHO road test on which most axle load equivalency factors are based, (v) operators of gravel hauling trucks consistently ignore axle spacing requirements, and (vi) in many cases, tire size rather than axle weight limit is the governing factor in determining the maximum legal axle weights.