Platoon, or just carpool?


At the recent AUVSI/TRB symposium, a popular research topic was platooning for robocars and trucks. Platooning is perhaps the oldest practical proposal when it comes to car automation because you can have the lead vehicle driven by a human, even a specially trained one, and thus resolve all the problems that come from road situations too complex for software to easily handle.

Early experiments indicated fuel savings, though relatively modest ones. At practical distances, you can see about 10% saving for following vehicles and 5% for the lead vehicle. Unfortunately, a few big negatives showed up. It's hard to arrange platoons, errors can become catastrophic multi-car pile-ups, other drivers keep inserting themselves into the gap unless it's dangerously small, and the surprising deal-breaker that comes from the stone chips which are thrown up by lead vehicles which destroy the finish -- and in some cases the radiator or windshield -- of following cars. They can also create a congestion problem and highway exit problem the way existing convoys of trucks sometimes do that.

One local company named Peloton is making progress with a very simple platooning problem. They platoon two (and only two) trucks on rural highways. The trucks find one another over the regular data networks, and when they get close they establish a local radio connection (using the DSRC protocol that many mistakenly hope will be the standard for vehicle to vehicle communications.) Both drivers keep driving, but the rear driver goes feet-off-the-pedals like a cruise control. The system keeps the vehicles a fixed distance to save fuel. The trucks don't mind the stone chips too much. Some day, the rear driver might be allowed to go in the back and sleep, which would allow cargo to move 22 hours/day at a lower cost, probably similar to the cost of today's team driving (about 50 cents/mile) but with two loads instead of one.

Trucks are an easy win, but I also saw a lot of proposals for car platoons. Car platoons are meant to save fuel, but also to increase road capacity. But after looking at all the research a stronger realization came to me. If you have robocars, why would you platoon when you can carpool?. To carpool, you need to find two cars who are going to share a long segment of trip together. Once you have found that, however, you get far more savings in fuel and road usage if the cars can quickly pause together and the passengers from one transfer into the other. Then the empty car can go and move other commuters. This presumes, of course, that the cars are like almost all cars out there today, with many empty seats. When the groups of passengers come to where their path diverts, the vehicle would need to stop at a transfer point and some passengers would move into waiting robotaxis to take them the rest of the way.

All of this is not as convenient as platooning, which in theory can happen without slowing down and finding a transfer point. This is one reason that the carpool transfer stations I wrote about last month could be a very useful thing. Such stations would add only 1-2 minutes of delay, and that's well worth it if you consider that compared to platooning, this carpooling means a vastly greater fuel saving (almost 50%) and a much greater increase in road capacity, with none of the other downsides of platooning.

If you're thinking ahead, however, you will connect this idea to my proposed plan for the future of group transit. The real win is to have the computers of the transport service providers notice the common routes of passengers early, before they even get into a vehicle, and thus pool them together with minimal need to stop and switch cars.

A number of folks have imagined designing cars that can physically couple, which would produce very efficient platoons and not add a delay. The problem (aside from the difficulties in doing this safely) is that this requires a physical standard, and physical standards are much harder to get working than software ones. It requires you find a platooning partner who has the same hardware you do, rather than software platooning, which can work with any style of car. Automated matching and carpooling makes no requirements on the individual robocars and their design, which gives it the best path to success.

It is possible (though a bit frightening) to imagine a special bus which could dock to robocars to allow transfer of passengers at speed. Some of you may have seen that a Chinese company has actually built the formerly hypothetical straddling bus (really a train) that has cars drive under it. If you were assured a perfectly smooth road one could imagine a docking extension which could surround a car door of a perfectly synced robocar and allow transfer. I suspect that's all pretty far in the future.

Beyond the carpool

In a robocar world, we should see a move to having vehicles with fewer empty seats. This happens if more people use single person vehicles for their solo trips, and as carpooling and other technologies make sure that the 4 seater vehicles end up with more people. Indeed, if the carpooling works, that happens naturally. At that point one might say, "now's the time to platoon." There is merit to that, but it comes later, rather than sooner. At this later date we can be more comfortable with the safety, and have a greater density of vehicles making it more likely to find others vehicles ready to platoon. Of course, we'll also have more vans and buses on the road who can combine even larger groups, if you find groups with a lot of journey in common. Platooning is practical even for a few miles, while carpooling tends to need a longer amount of shared journey to make it worth the switch.

At that point in the technology, you can do much more serious platoons, with larger groups of cars, and distances which are short enough for even greater benefit, and short enough to strongly discourage people trying to insert themselves in the middle of the platoon.

So platoons will come and give us even more road capacity. Carpooling, though, is already happening, with 50% of Uber requests in San Francisco being done in UberPool mode. It is the more likely early answer.


Congratulations, you've invented self-driving public transit! ;)

Honestly, the fact that "50% of Uber requests in San Francisco [are] being done in UberPool mode" should make you seriously re-consider your oft-repeated contention of how much weight most people give to traveling in a vehicle all to themselves vs other tradeoffs. Humans *are* social creatures, after all. Civilization is built on this. If we couldn't handle being in (very) close proximity to other people we don't know for short-to-medium stretches of time (or longer), cities simply couldn't exist.

In fact, the amount of solo space, solitude, and alone-time that so many humans get in the modern world as they spend many hours driving alone in their four-seat car to commute to work, and living in homes where there's enough rooms that every family member can be entirely out of eye- and ear-shot of each other at any point in time, is quite unusual in human history. Any ideas that people will of course naturally prefer these conditions can only be borne of ethnocentrism from personally living in that kind of environment. (Of course there are many human cultures where it's simply unthinkable to spend that much time alone.)

...the surprising deal-breaker that comes from the stone chips which are thrown up by lead vehicles which destroy the finish — and in some cases the radiator or windshield — of following cars

I've only been bringing this problem up for the past umpteen-many years every time platooning gets mentioned. Yep, surprising. :O

It is not my experience that people do UberPool to be social. But then I have only used it 3 or 4 times, but generally it was just polite hello to the other passengers or light conversation, but more often the same staring at phones you see everybody doing on the subway.

I agree with Brad. I take both UberPool and Lyft Line - and most of the time there is no conversation happening in the car.

I think that most people like to take the pool options because they save money. Period.

I think the reality is that most people simply don't want to share cars. You only have to look at the success of car-sharing schemes in to see that, in general, people don't use them. I could give a list of reasons, but it would go on for some time eg. choice of music, cabin temperature, other people's bad habits, security, conversation, personal space, keeping belongings in the car etc. etc. The bottom line is that the personal car is, for many people, an extension of their own personal space...and they want it to be private.

On a slightly different note, with regard to platooning, why does no-one ever factor in the wage and responsibility of the 'lead driver' - the lead driver would have to be a trained professional (even then there is no way would I hook my family up to one of these things), and that professional is going to need paying; the wage and costs of this professional would probably negate any fuel savings for the remaining vehicles, plus it would also mean yet more traffic on the road as you now need to account for the professional driver's vehicle as well.

(Note that sharing cars and sharing rides are two very different things)

The surprising truth is people are willing to share rides to save money, and get access to carpool lanes. I suspect that as it takes off, you will see cars designed for sharing, for example, with a privacy divider in the back.

The Sartre platoon plan had the lead driver always be a trained truck driver who was hauling a load, and cars and other trucks could attach on the back. No extra pay needed -- even the lead truck saves fuel.

However, I am talking about robot platooning, no drivers.

I think that the question of whether people want to ride in an empty or full car is premature. Probably there are some of each (especially if there's an easy to read meter running or the worst of city bus patrons present). But we never even get to that because of coordination. We have had HOV lanes for decades and I do not get the sense that they have provided incentive to completely restructure people's transportation habits in a revolutionary way. The problem is that if you need your own car for that last mile to a weird place or to carry those 14 bags of groceries you just bought or to take your dog to the beach, etc, car pooling is just not even something that one can consider.

I also just did a week vacation from S.California to Colorado. I flew, but it took all day to fuss with the TSA, awkward packing, rental car, and final destination far from the airport. If I could platoon my car and sleep for half of such a drive, that would beat flying for me on such a trip.

One important difference between platooning and carpooling is the ability to carry luggage. You discuss the shifting around of people from car to car, and how straightforward that would be even for personal automobiles. But it's a trickier situation when there's a lot of luggage involved, as is commonly the case for long-distance driving. Platooning handles this aspect more gracefully.

Agreed. Mostly we're talking about commuting here. Carpooling with luggage is still a lot easier than riding transit with it.

For long haul trips, where you do have luggage, it is a burden. But it's also a very big win there, particularly as luxury coach travel becomes available. In today's cars which cost about 50 cents/mile, a 300 mile trip has sufficient cost that most people would handle the burden of a luggage switch with a 10 foot walk to take a $50 luxury bus or $40 carpool.

Platoon savings are so tiny compared to carpool savings, but people love convenience, it's true. That's why super-convenient carpooling is interesting.

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