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Whither Platooning -- and talk at Dorkbot tomorrow


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.


Hi Brad

Am in agreement with all of your sentiments. The more I think about platooning as a concept the less I believe in it. There's just way too many obstacles to overcome and the ROI appears negligible.

Just a thought. one of the advantages of automation on the roadway will be a greater capacity to monitor and mediate roadway conditions---the robotic street sweeper for example. When cars undergo the fundamental transformation to non-human control the greatest change will be in the shape and nature of the roadway. Smaller, faster, lighter and.....cleaner, I would think.

The stone chip problem is interesting, and I certainly wasn't expecting it.

How do the stone chips end up on the roads in the first place, and is better highway cleaning an option if we care about letting vehicles follow closely?

(I'm reminded of how Amtrak concluded within the last few years that investing in platform washing equipment for certain stations along the route of the Cardinal would be worthwhile; I think they concluded the staff that they already had could do the cleaning, and that this would result in a lot less coal dust being tracked into train cars.)

This might also be an opportunity to develop better windshield materials.

Or perhaps there should be an outer windshield that can roll up in front of the inner windshield when you're following closely.

Regarding traffic lights, there are already places where mass transit vehicles get signal priority. Platoons could potentially get similar priority. It does violate the ``don't change the infrastructure'' principle slightly, though.

I am not sure the test roads were all that dirty. I think small bits of stone are always finding their way onto roads (or coming off of roads) and they get thrown up but just fly a few feet -- not a problem unless you are following just a few feet behind.

The problem is that if people do this with their own private cars, their shiny new cars, they are not up for ruining the windshield and finish of the cars. If you do it with cars dedicated to this purpose, you might just view it as part of the cost of the system. After all, it does give you something really good, the ability to kick back and read while on your commute or long trip. If you buy a car just for the commute -- as many people do -- you might just expect that such a car is going to have a less than smooth appearance.

What it means is that if you are going to join a convoy, you will be making a decision to dedicate that car to a lot of convoy operations.

Delaminated heavy truck tire treads,
and dropped items like ladders and gravel rakes,
can cause much worse damage than chipped windshields.

Which report cited stone chips as the only problem?

Were actually done on closed test tracks. All roads have small amounts of debris which they got on the closed tracks. You could be right about other stuff, but I don't know if it flies quite as high to hit your windshield. You probably won't be following a truck in a convoy that drops a ladder... It might encounter one on the road, however.

The stone chip problem is interesting but it seems way too soon to consider this a serious problem.

There's an opportunity for a careful study of how the stones get kicked up and what can be done to suppress them. A great Master's thesis to be had here!

Most trucks have mud flaps. You mention that the forward vehicle is kicking up the stones. Does this mean the lead truck? If so, did it have mud flaps? If so, could better mud flaps do a better job?

Car tires also kick up stones. Few cars have mud flaps, but they certainly could be equipped with them to participate in convoys.

Also, the following distance matters. 4-6 meters seems like a lot. With closer following, the stones may not get so high before they hit the front of the next car. I thought that at one point you had suggested that closer following is preferable not only for fuel efficiency but also because the opportunity for relative velocity to develop during a crash/panic situation is diminished.

They experimented with all sorts of following distances. Different distances offer different gas savings (10% to 30% but there were some tests where they got worse gas mileage so it's pretty complex and depends on vehicle shape.) In Volvo tests it was always a truck in the lead, and generally trucks must go at the start of convoys due to longer stopping distance.

Cars are always getting hit by small chips and grains and stones on the road, but at typical following distances real damage is rare. The main report from the convoy researchers was that after lots of convoy driving, they found problems with stone chips higher than people would like. Not all chips give you cracks and dings -- you hear them all the time even in normal driving. It's just that following close bumps your number. Possibly there could be work on proper flaps or other techniques to reduce them.

Really close distances are also disconcerting to occupants and require closer tolerances and have greater risk if there is failure.

Re: Really close distances are also disconcerting to occupants and require closer tolerances and have greater risk if there is failure.

I guess I don't see a difference between following at four meters (about 12 feet) or one meter. In either case, if a bad wreck happens the collision will be the same. Won't it?

An analysis that determines probabilities to all the possible accident scenarios, and then estimates the severity at different following distances, would be valuable. I am not aware even of preliminary studies of this type.

However, we can make a couple of observations and perhaps gain some insight:
1) At the largest following distance of interest, the followers would be able to meet the so-called BWS requirement. BWS stands for Brick Wall Stop: if the vehicle in front suddenly hits a brick wall, then its follower must be able to make a complete stop without colliding. (BWS does not posit how the "brick wall" suddenly appears in the roadway. Obviously heavy rush hour freeway traffic does not meet BWS.)
2) The shortest following distance is zero, a situation where the bumpers are touching. In this case, which I have considered extensively, it would be best to have the cars couple mechanically as well.

By definition, at the BWS following distance, followers incur zero damage. If connected, the damage will tend to be mitigated by several factors. One is that there can be no secondary rear-end collision, because cars that are already touching cannot collide. The connected cars will tend to act like a more massive vehicle with greater momentum. Much as an SUV is safer than a small car in a crash, the greater mass can be protective. Followers will also benefit from a much longer crumple zone. Note however that the passenger compartment must be designed to protect occupants from what could be a crushing mass of follower vehicles.

As the following distances increase from zero, the crash analysis becomes more complex. There be not one collision but a series. Except fot the last cars, each will suffer an additional rear-end collision that introduces the possibility of whiplash injuries.

This rudimentary analysis suggests that the graph of average collision severity will have a peak between zero and the BWS following distance. The average accident severity eventually declines because the followers approach the BWS distance and can slow down by braking.

Assuming fully enabled radar braking, the BWS distance is on the order of 100 meters at highway speeds. By comparison to no collision at BWS, the difference between following distances of one and four meters may seem slight, but there are likely to be quite significant differences. As the separation between vehicles increases from zero meters to four, the average intensity of the follower collisions will tend to rise steeply.

To keep informed about zero following distance, mechanically coupled “platoons” or Hybrid-Electric Roadtrains (HERs), sign up at

Check out these reports:
Hitchcock, A., "Layout, Design and Operation of a Safe Automated Highway System", UCB-ITS-PRR-95-11, April, 1995.
Hitchcock, A., "Intelligent Vehicle Highway System Safety: Multiple Collisions in Automated Highway Systems", UCB-ITS-PRR-95-10, April, 1995.
Alvarez, L., Horowitz, R., "Safe Platooning in Automated Highway Systems", UCB-ITS-PRR-97-46, December, 1996.

"All this has happened before, and all this will happen again" (tip of the hat to Brad on that one!)

One of the safety features in the design back then was the presence of barriers that would prevent cars from crossing into the platooning lanes, but would have enough give when hitting them on end to not kill drivers in gate traversal accidents. So, some of the factors will be different this time around.


If the cars were on rails, this hypothesis about the damage at various distances would be true.

However, they are not on rails, and in reality are on a multi-lane road (or even road with a shoulder) that offers other avenues of escape some of the time. In very dense traffic, which is the worst case scenario, there are fewer avenues of escape, though rarely none. Non-platooning cars, even following quite closely, tend to leave more than a car length between them most of the time, and almost always when at speed.

This, by the way, is where I actually see some value in the use of vehicle to vehicle radio links, a technology that is otherwise of only modest value. Cars don't need to communicate to take evasive action when they have radar/laser tracking on other vehicles that tell them velocity and position, because they can very quickly see other cars changing speed or moving laterally. What the communication can do is allow you to judge whose software is running the other car (if any) and if you can trust it to coordinate with you.

So if the car to your left starts moving into your lane to avoid a collision, you don't need a radio to start moving right if you have a gap. If you don't have a gap you now have a problem. You could move to the right and hope the car to your right will react quickly and give you the gap, but it might not. However, if the gaps exist, you can also brake quickly so that a gap appears to your right and you can enter it. If you can communicate you can negotiate a strategy for all the communicating cars to open gaps and move away from the collision.

This is not to say that it's considered safe to suddenly move into a gap that's not very large. It is just vastly safer than hitting the car in front of you is! Humans often avoid accidents by doing unsafe things, and it usually pays off. Sometimes it doesn't but overall its the right strategy. There will be an issue when it doesn't pay off and this harms somebody who wasn't even in the lane. One hopes the law will accept that certain moves, designed to reduce the risk the most may also redistribute the risk, and bring harm to those who would have stayed unharmed in the "do nothing" case. The fault is still, I feel, with the people who caused the accident, not the systems or people who moved to avoid it and caused other accidents. But we'll see how the courts rule.

If traffic is only modestly dense, it should be possible to escape most bad situations. The one that might not be escaped would be cars crashing and moving sideways to block multiple lanes. To avoid those you need the brick-wall-stop distance, and frankly, motorists seem unwilling to leave that level of safety margin. In fact, if you programmed a vehicle to leave that safety margin in any noticeable amount of traffic, the gap would constantly be filled by people cutting in, making travel very difficult, and platooning impossible. Platoons need to operate with a gap too small for people to cut into it, in fact if they leave the physical room, some may still try to use it and techniques (honking, closing) may be needed to keep the cut-ins out.

When convoys pass convoys on the highway, it may well be that they would spread out while passing one another. During such passes, solo cars can't cut in to the longer gaps, and the longer gaps allow for better handling of any calamity during the passing. One might ask if convoys should ever pass convoys but I am sure it will be desired.

Musing (and without your expertise), I wrote a blog piece hoping it would work:

it does not get around all of your issues, but recent tech might make some of the issues of semi-autonomous action within easy add-on range for new cars.


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