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14 Figure 11. M.o.U. tandem semitrailer. Figure 13. STAA twin trailer. the latter shown in Figure 11. The tridem semitrailer, shown did require that all provinces allow the configurations it defined, in Figure 12, introduced a configuration with an intermedi- with their dimensional restrictions, at the specified weights. ate weight capacity. The 1988 M.o.U. (1) was similar in form to the current Short and medium length A-train doubles like that shown version (34), which is reproduced in Appendix A. However, in Figure 9, were found to have undesirable vehicle dynamic the 1988 M.o.U. only included specifications for a tractor- characteristics, and, therefore, these vehicles were limited in semitrailer, and A-, B- and C-train doubles, and some of the the allowable gross weight of 53,500 kg (117,946 lb), though detailed provisions for these configurations were different this limit still allowed for unrestricted access by the U.S. Sur- than the current version. The increase in overall vehicle length face Transportation Assistance Act (STAA) twin trailer, shown from 23 to 25 m (75 ft 6 in. to 82 ft) was a concern and was in Figure 13. The allowable gross weight of C-train doubles reviewed in detail (34). While there were issues with a number was provisionally limited to 53,500 kg (117,946 lb), pending of traffic standards, these issues were also present for current further work to define key properties of the C-dolly. The 8-axle vehicles. In particular, while sight distance and road marking B-train double, shown in Figure 14, was found to have excel- standards were questionable in passing situations, drivers lent vehicle dynamic characteristics with a gross weight of interpreted the road markings in a much more conservative up to 62,500 kg (137,787 lb). With the weight advantage, it manner than the standard (36). immediately became the configuration of choice for heavy loads in the four Western Provinces. The box length of A- and 2.1.6 Implementation of the M.o.U. C-train doubles was limited to 18.5 m (60 ft 8 in.), while the B-train double was allowed 20 m (65 ft 7 in.), to encourage Each province had the freedom to designate what portions use of B-trains for light- and moderate-density freight. A num- of its highway system the M.o.U. configurations would oper- ber of key internal dimensions were specified for each config- ate. British Columbia and Ontario each designated their entire uration, to ensure that they met bridge loading and dynamic highway system, because their prior rules already allowed performance standards. vehicles at weights no less than those specified by the M.o.U. It is important to note that the M.o.U. did not require that all Alberta, Saskatchewan, and Manitoba each had significant provinces adopt these configurations as their only vehicles. It amounts of highway with thin flexible pavement, or second- Figure 12. M.o.U. tridem semitrailer. Figure 14. M.o.U. 8-axle B-train.