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10 Figure 3. A-train double, 6.1-m (20-ft) trailers from the 1960s. either to roads or bridges that would have been expected. Figure 1. Tandem semitrailer from the 1960s. The Ontario Department of Transport undertook a series of studies of the load carrying capacity of existing bridges on the basis of the traffic observed in the load survey (4), (5). This between eastern and western Canada was through the U.S. resulted in the development of the Ontario Bridge Formula Uniformity between provinces and states was also not a big (OBF) as a safe operational load limit for bridges (6). issue, as cross-border trucking was much more restricted The OBF introduced the concept of greater allowable than now. weight on an axle group with greater spread and became the means to control axle weights in Ontario, especially those of 2.1.2 The Ontario Bridge Formula, 1970 the short heavy vehicles mentioned above. The allowable load on a single axle was obtained from pavement considerations, The Ontario trucking industry campaigned for higher legal and the formula extended this to provide a safe load for weights through the 1950s and 1960s to improve the compet- bridges for a group of consecutive axles on the basis of the itive edge of Ontario truckers, and it won increases in 1960 spacing between the axles. The formula allowed an increase and 1966. Studies by the Ontario Department of Economics in axle loads of about 10% over the prior law, as follows: and Development in 1966 concluded there was no economic basis for an increase in the single axle load (3). The majority 9,071 kg (20,000 lb) for a single axle; of weight enforcement at that time was through the weighing 15,875 to 17,690 kg (35,000 to 39,000 lb) for a tandem axle, of an entire vehicle on a large platform scale for gross weight. depending on spread from 1.22 to 1.83 m (48 to 72 in.); and The Ontario Department of Transport conducted a survey of 19,958 to 27,215 kg (44,000 to 60,000 lb) for a tridem axle, truck size and weight in 1967 and found significant overload- depending on spread from 2.44 to 4.88 m (48 to 192 in.). ing of axles on vehicles that were within or only slightly over their allowable gross weight. It also found that a lack of axle The bridge formula resulted in a corresponding increase in spacing control in the then-current law had resulted in a large allowable gross weight, to a cap of 63,503 kg (140,000 lb), proportion of very short trucks with closely spaced axles that though it was difficult to reach this as the maximum overall could be very damaging to bridges. However, these vehicles length remained at 19.81 m (65 ft). and their overloading did not appear to cause the distress The OBF was introduced into the Highway Traffic Act (HTA) in 1970 to govern axle group and gross weight. Motor carriers persuaded the ministry that they needed some time to adjust to the new weight regulations, so a regulated toler- ance was introduced for axle group weights. This initially allowed 1,587 kg (3,500 lb) per axle for a single, tandem, or tridem axle, but the tolerance decreased at 227 kg (500 lb) per year to 454 kg (1,000 lb) per axle by 1975. The OBF was simply a means to evaluate a vehicle, and it did not control axle arrangement or vehicle configuration in any way. Vehicle designers quickly mastered the formula, and developed a variety of axle and vehicle configurations to max- Figure 2. Tridem semitrailer from the 1960s. imize the allowable gross weight. These included semitrailers