Here's how a road train works: the convoy is controlled by a lead vehicle with a professional driver at the helm--one day, this is where all Formula 1 retirees will end up. The other cars communicate with the leader to join and leave the train when they want, thanks to wireless sensors and their existing sat nav systems. Once on the convoy, the drivers behind the leader are able to take their hands off the wheel to read a book, watch TV, or check company sales figures and decide which poor minion is for the chop this month.
In theory, this is supposed to cut down on congestion—if more cars are traveling at a constant speed, those traffic jams that materialize out of nowhere are less likely to form. (Of course, it's possible that congestion would form as cars slow down and pile up to try to join one of these road trains.) It could also cut down on gasoline use. Though, as Tom says, one downside is that drivers could have trouble shifting from "passive" road-train mode to "active" driving—with deadly consequences.
In any case, as a commenter at Fast Company points out, this was all predicted back in 1958 by the Disney TV show "Magic Highway USA":