According to insurance industry magazine Claims Journal, the proposal appeared in the DOT's March 2014 "Report of Significant Rulemaking." It would force big rigs weighing over 26,000 pounds to use speed governors with an unspecified limit. Many trucking companies already fit their tractor trailers with limiters, but they aren't obligated to. This isn't the first attempt at putting a max speed on semis. Organizations like the American Trucking Association previously suggested a 68 mph max for new trucks.
Other truckers aren't so happy about the possible change. According to Claims Journal, the Owner Operator Independent Drivers Association has come out against the new rule. It argues that speed differential is the dangerous factor, not outright velocity, and believes that it's better to keep all traffic traveling at a roughly equal rate. Also, if all tractor trailers are governed to the same speed, it makes passing very difficult, the association argues.
By lowering the semis' speed, the DOT hopes to curb accidents by reducing the trucks' stopping distance. There is currently a big push to make big rigs safer. The National Transportation Safety Board and National Highway Traffic Safety Administration have recently published independent sets of suggested regulations for them. NHTSA also wants to mandate stability control on tractor trailers. Other countries that require limiters note that slower speeds also improve fuel economy.
Keep in mind, this is still a proposal and in the earliest stages. Claims Journal says the rule first has to be approved by Secretary of Transportation Anthony Foxx and then cleared by Office of Management and Budget in the White House. If it makes it that far, it could be published as a Notice of Proposed Rulemaking in October. According to the DOT's website, there would likely be about 60 days of public comment. From there, if published, it couldn't go into effect for a minimum of 30 days. That is a lot of time for things to change and a lot of hurdles to jump over before any tractor trailer is fitted with a mandated governor.