Road trains (also called vehicle platooning) are convoys of semi-autonomous vehicles with a professional driver in the lead vehicle. The Safe Roads and Trains for the Environment initiative (SARTRE) describes road trains as:
…a convoy of vehicles where a professional driver in a lead vehicle drives a line of other vehicles. Each car measures the distance, speed and direction and adjusts to the car in front. All vehicles are totally detached and can leave the procession at any time. But once in the platoon, drivers can relax and do other things while the platoon proceeds towards its long haul destination.
Road trains were actually tested in the real world by Volvo, who is part of the SARTRE team, in December. They cite the benefits of road trains as numerous:
Platooning is designed to improve a number of things: Firstly road safety, since it minimises the human factor that is the cause of at least 80 percent of the road accidents. Secondly, it saves fuel consumption and thus CO2 emissions by up to 20 percent. It is also convenient for the driver because it frees up time for other matters than driving. And since the vehicles will travel at highway speed with only a few meters gap, platooning may also relieve traffic congestion.
There are some potential downsides to road trains as well, but ideally they can deliver many of the benefits of intra-city transit without some of the drawbacks. Really road trains are just a stepping stone to fully autonomous cars, and caveats of same apply here as well.