A proposal is being floated in Europe for computerized convoys or road trains within the next decade. This is a proposal for a system where cars can hand over control to a lead car and follow in a train or convoy, without physical connection.

This idea comes up a lot as an early robocar technology. It is particularly common because it's much easier to do -- a human driver still is in charge, and the robotic control is limited to a very limited and simple environment. It's safe to say that we could make this work very quickly if we wanted to. There is no navigation or vision required, no recognition of obstacles, no choice of speeds or turns. Cars that come together in a convoy can draft to get a serious boost in fuel efficiency, and of course the un-drivers can now relax and read or work on the trip.

As a robocar booster, people are surprised when I say I am not too thrilled about this idea, at least as an early technology. Rather I think it's a great idea for later. In spite of the enthusiasm with which I write, the robocar problem is not a simple one. This much simpler problem is tempting but has some snags.

First of all, if you have a bug in a standalone robocar system, it may cause an accident, and that may injure or kill the occupants of the robocar, and perhaps one or two other cars. Death is less likely at urban speeds of course. A problem with a computerized convoy could have terrible results, involving scores of cars. Since most people want this for the highway, the problem would also occur at lethal speed. Convoys are just not the first place we want to test our systems and have our first accidents.

Secondly, forming convoys requires a critical mass of suitably equipped cars. Of course, you don't need a dozen full robocars to make a train, all you would need is cars with drive-by-wire and some much simpler control circuitry. But even so, the incentive to get a car with this feature has to get over a critical mass hump if it's going to be worthwhile. It's not quite as bad as fully ad-hoc trains, since you can have scheduled trains, lead by a bus or truck driver, and cars can see such a lead vehicle and get in behind it. But at first, the odds of many cars all finding one another at the same time is low. If the train is going faster than regular traffic in a carpool lane, as we hope it would, it will not be easy to join a train that moves past you on the highway. If it moves slower than traffic, it is easy to slow down and join it, but then it has to move slower, with all the attendant problems.

Computerized convoys have advantages and disadvantages over physical ones. Physical ones probably can only be formed while stopped, and probably only unformed that way too. One could see the last car in a physical convoy undocking while moving, so with correct ordering it might work out, but it's a far cry from a virtual convoy which allows anybody to join and leave at any time.

Physical convoys however can transmit power. This is useful if you expect people to be driving short-range electric cars. They would take their short range car and join a convoy, and be powered by the lead locomotive while operating, and even be recharging a bit. After dispersion, the vehicles would only need to go a short destination to their target and back to the evening train.

Physical coupling makes it harder for one car to leave the train due to a failure. On the other hand it means that if the lead car wants to change lanes, all cars must do so. If the lead car leaves the road, they all do. Jack-knifing is a real worry, which is one reason that today even cargo road trains are limited to 2 trailers in urban areas, and 3 trailers in rural areas, if they are allowed at all.

Physical coupling requires specially modified vehicles. This is even more the case if the locomotive will actually be towing the vehicles physically rather than providing them with electricity for their motors and batteries. Either of these is a major modification, while virtual coupling only requires a drive-by-wire car and a small matter of programming.

Even full robocars probably should not form convoys right away. We should wait until our confidence is even higher, in spite of the fuel savings. If one car goes bad, or its occupants try to take over and move to manual driving, the consequences could be nasty in any convoy. And of course, the first robocars on the road will never get to join convoys as they will not meet the others. That's why you need to solve the solo navigation problem first, and then you get enough on the road to work on the cooperation problems.