Tomorrow (Wed Sep 19) I will give a robocars talk at Dorkbot SF in San Francisco. Dorkbot is a regular gathering of "People doing strange things with electricity" and there will be two other sessions.

Last week, the SARTRE project announced it was concluding after a long period of work on highway platooning. Volvo lead the project which demonstrated platoons on test tracks and on some real roads. They also did a number of worthwhile users studies in simulation.

People have been interested in platooning for a while. The main upsides they are looking for are:

• It's much easier than a Robocar -- the platoon is lead by a truck with a professional driver who handles everything with human intelligence
• Putting the car at short spacings can result in a huge increase in highway capacity, though you tend to want somewhat larger headways around the convoys
• There is fuel saving -- about 10% or so for the lead vehicle, and up to 30% for following vehicles, at spacings of about 4 to 6m. This is not quite as much as people hoped but it is real.
• The equipment in the following cars is simple -- V2V radios and possibly some radar for backup.

Unfortunately, platooning comes with some downsides as well

• If you have an accident, it can be catastrophic as you might crash a whole convoy of vehicles.
• Non-platoon drivers may interfere with the convoy. The gaps must be kept small enough that nobody tries to enter them. A non-member in the middle of the convoy is bad news. You need small gaps to save fuel too.
• Trucks must go only at the front of the convoy due to their longer stopping distance. New trucks must insert in the middle. Cars can insert more easily at the end of the convoy.
• Convoys in the right lane can make it harder for people merging, and in general they can present a barrier to traffic.
• Driving with a short gap is disconcerting. Behind a truck, you can't even see the lane markers.
• In rain, your windshield gets completely washed out with spray (and sometimes salt spray) which is even more disconcerting.
• Following cars get hit by small stones and debris from the forward vehicle. After a long period of following, windshields are unacceptably chipped or cracked.
• While radar is the primary means of tracking the car in front, and almost all vehicles do a nice radar reflection from the rear licence plate, many vehicles have other reflections further forward. You must avoid trying to follow 4m behind the front of a truck! To help this, vehicles in the tests had superior radar reflectors mounted on them.

For good workable convoys, some of these problems need to be solved. It could be that in rain convoys must spread out (losing a lot of the fuel saving) though there is the danger of cars cutting in.

Convoys with longer gaps can still increase road capacity a lot, but they probably have to be robocar convoys. Robocar convoys can handle cars trying to cut into the gaps. They may wish to start honking if somebody cuts-in (and the car in front might also flash its rear lights and slow slightly to make it very clear to the cut-in that they should not have done this.) This would be a problem when convoys are new, as people might not know what it all means, though they would have tried to go into a space that is clearly too small to safety enter. Cars in convoys might need to have a screen on the back that can display a sign "You have barged into a convoy, change lanes immediately or be reported to police."

Robocars could handle the rain to some degree, but even their laser sensors would not like operating in heavy spray, though their radars would get excellent returns from a reflector on the vehicles.

The stone chip problem is harder to solve. Robocars capable of full auto operation could try to protect their windshields, but this is disconcerting to occupants. And the rest of the car gets stone chips too.

It could be that platooning is only practical with vehicles that are dedicated to it, such as highway commute vehicles and long distance highway vehicles. Built for this purpose, they would just accept the stone chips as part of life. They might come with extra heavy duty wipers or other ways to deal with the rain. And they would be full robocars, able to handle disconnects and independent operation.

This result will disappoint those who felt platoons were a good early technology. I have felt they also suffered from a critical mass problem. To use a platoon, you would need to find one, and until the density of lead vehicles was high enough, you might not find one. You could do it at rush hour with mobile apps that track the presence of lead vehicles so you can time your departure to find one -- you might even have an appointment for every commute. And they might run only on nice clean highways on dry days and still be valuable. But less valuable, I am afraid.

On lower speed roads the fuel saving is not much, but the problems are less. There are traffic lights on most low speed roads though which present another problem.