For everyone's information, here's a link to a simplified map I was able to locate:
http://www1.eere.energy.gov/vehiclesand ... tw411.html
My first "real" (after-college) job was in the Central Dispatch office of the long-departed Jones Motor, so I can recall some of the issues of the day.
By the early 1970's, "double-bottom" rigs were allowed in all states except for a handful along the Eastern Seaboard. Massachusetts, Virginia and Pennsylvania were viewed as the most restrictive. However, the "doubles" were of trailers (sometimes called "pups") of no more than 23 feet in length rather than the present day 28. Only one state (Nevada) allowed "triples" in those days, with a maximum length of 105 feet -- power included.
The 45-foot semitrailer was legalized in all states by the summer of 1971. The 40-footer had ben standard for a long time before that (and of course, the 88-foot TOFC flat which dominated things in the 60's was designed on that premise), but I can't say whether the 40-footer replaced a 35- or 38-foot standard, and information from the web that far back is sparse. With the completion of tunnel-free and toll-free Interstate 80 across Pennsylvania in the fall of 1970, the presssure to ease restrictions grew. Industry leader Consolidated Freightways came up with what we used to refer to as a "Pennsylvania double", essentially a straight truck with a 23-foot box pulling a full trailer of the same size. According to some sources, the rig could be easily reconfigured to the "traditional" double once the Ohio line was crossed, but I never saw proof of this.
That limit didn't include what we referred to as "trains" ... two full size trailers ... which were permitted on the New York Thruway, Massachusetts Turnpike, and the Ohio and Indiana Turnpikes. Those rigs required special larger tractors, and the drivers had to be specially licensed by the toll road operators. Putting them together was accomplished at staging areas at some of the interchanges; the limits were such that a 45-foot box always had to be twinned with a 40-footer. Pennsylvania wisely refused to participate on either the Turnpike or the "Shortway" (I-80) due to the tunnels, grades and curvature, but it's intersting to speculate on what might have turned out differently if only that flat, straight section of I-90 along Lake Erie had been excepted.
Resistance to larger truck sizes and weights by individual states was finally overcome in the early 1980s, and just as with the effort to impose a uniform 21-year drinking age, the threatened witholding of Federal highway funds was a key component of the equation. The acceptance of the 28-foot trailer and the use of "triples" also came into full flower at this time, as evidenced by the linked map.
In retrospect, while the industry mght have succeded in getting the weight limits it sought, the same period saw both a large shrinkage in the number of carriers, and the practice of local pickup/delivery and terminal-to-terminal line-haul in the wake of deregulation. Very few of the truckload-based carriers who replaced most of the familiar names of the 1970's make use of double- or triple-trailers, and Roadway Express and Yellow Freight, two of the formerly-dominant firms, merged out of necessity and are reportedly being kept afloat by their bankers while restructuring similar to that which created Con-Way out of Consolidated Freightways can be effected. Only ABF, which also absorbed successful Carolina Freight about 15 years ago, seems to be holding its own.
Looking back, if 1973 was a poor time to go railroading, it was a good time to be young and working for a dynamic competitor. But nobody thought things would turn out the way they did.