The idea that sharing rides is good has become almost axiomatic in transportation discussions. At conferences I have seen people declare that robocars are pointless if they are not shared -- ie. people who are not travelling together ride together in them. The positive of sharing is so axiomatic that public transit is seen almost as a good in and of itself, rather than a means towards real goals like energy efficiency, low cost, and higher road utilization.

It has has attained this status as revealed truth because it is indeed roughly true -- more people together in a vehicle done right will indeed use less energy per person and less road space. But the "done right" is very important as it is commonly done quite wrong.

As I have studied robocars, this has led me to the discovery that some of our old assumptions are wrong. In particular, more sharing is not always good, and the styles of sharing (including the vehicle sizes) of current public transportation are almost certainly not the optimum sizes, and that smaller vehicles are likely more optimal once we eliminate the need for drivers and move to a highly communicating world.

I believe there are strong arguments that while shared travel is beneficial, we actually have too much of it in most transit systems, and not enough in private cars. That the "shared" future is one of van-sized group vehicles with a mixed fleet of more personal cars with 1-4 seats.

When is sharing bad?

Sharing creates negative factors because it does not exist alone in a pure world. Shared transportation competes with personal transportation and other forms. In the real world, travellers have different needs and income levels.

Almost all sharing today comes with some negative trade-offs for the passengers. Riding a shared vehicle can mean:

• Leaving or arriving at a time different than they desire
• Taking a route that is other than the most direct one, with a longer travel time than that route
• Severe delays and inconvenience when transferring between routes
• Making stops for other passengers
• Using a slower vehicle due to its large weight and size
• Having to stand for some or all of the journey, or getting an uncomfortable seat
• Getting packed next to people they would prefer not to travel with

Generally, the risk of all these factors goes up the more people share the vehicle, but more to the point, the "aggregate pain" always goes up and in fact can go up faster than linearly with the number of people.

The greater this pain, the more people are inclined to find alternate means of transport, including car transport.

Small vs. Large

A simple thought experiment can show how more sharing can fail. Imagine you have one train every 15 minutes. You could replace it with a much bigger train (but not 4x bigger) every hour, or an even larger one every 2 hours. That's clearly more efficient, but it would inconvenience riders so much that a large fraction of them would abandon the train entirely. Transit planners know that transit lines founder when frequency is too low.

A more real example involves buses. Many bus lines might have one 50 person bus every half hour. But riders would find the bus line far more useful if it had a 25 person bus every 15 minutes, and vastly better with a 10 person vans every 5 minutes. The latter are a bit less efficient, but the main reason this can't be done is that today the cost of bus drivers is a major faction of the cost of the system.

Those 6 vans might seem to take up more road capacity than the large bus, but even that is not so clear. A 10 person robotic van need be no larger than today's sedans, and those are much less disruptive of traffic than large buses. In addition, the van would pass by many stops that the large bus must stop at. Stops are wasteful of energy and disruptive of traffic. The 6 vans have more capacity than the one bus, and they will need it -- because quite a few people would find the 5 minute service much more acceptable.

The capacity of heavy rail and LRT compared to Bus Rapid Transit is also affected by the removal of drivers. Heavy rail might have 7 to 11 cars. An LRT has 4 cars in it, while a bendy bus can have 2, so per driver, the bus capacity seems lower. Without drivers, the bus capacity is higher because buses can stop "offline" -- without blocking the track -- so capacity can be almost arbitrarily high.

Another example of the failure of large vehicles can be seen recently in commercial aviation. Airbus produced the giant A380, based on the idea that fully loaded, it would be very efficient. A380 service usually means there is just one flight a day on the long routes it serves, causing maximum inconvenience for passengers. It also takes longer to load and unload. The A380 is now viewed as a mistake, with airlines all moving towards smaller, lighter aircraft like the 787 and A350, which allow more frequent service and more nonstop service. In 2015-2017 there were net zero orders of A380s, though Emirates ordered some more in 2018.

Load Factor

Another big factor is the load factor of the vehicles. Since you don't have an infinite budget for vehicles, you have to pick a fixed set, and when you have large vehicles, you use vehicles which are much too large for the load some of the time. The average bus in the USA has just 9 passengers on it. It's pretty efficient at rush hour, standing room only, but then it gets re-used off peak with a very light load. This is no minor thing. A bus system with 4mpg large buses gets 36 passenger miles per gallon in total efficiency. This is worse than a single person driving a 50mph Toyota Prius, and much worse than a Prius with 2 people and even worse than an electric car. Light rail vehicles, massive as they are (the "light" stands for light capacity, not light weight) do even worse.

Current transit designs demand not just tightly packed, efficient vehicles at rush hour, but large empty vehicles in the reverse commute direction and at off-peak hours. Riders will not usually accept a system that can't get them home except at rush hour. (Certain commuter rail lines are the exception.) This drops the total efficiency way down.

Smaller vehicles offer a solution because you can more closely size the vehicles to the load. If there are only 8 riders, you send a van or two cars, not a 40 person bus. In the above bus line, late at hight you can drop the service to every 15 or 20 minutes with small vans, and still get decent load on them while offering better service than the large bus every 30 minutes.

It will surprise many to learn that the US Dept. of Energy reports that bus and light rail systems of the USA with just a few exceptions use more energy per passenger-mile than average cars, and far more than efficient cars like hybrids and electrics, which just have an average of 1.5 people per car. That the optimal sharing point is on the small end rather than the large, at least in the USA, becomes very clear.

This is even true of the most efficient system in the USA, the New York MTA Subway. Most recent figures suggest it uses about 180 watt-hours/mile/passenger. The Tesla model 3 is 240 watt-hours/mile, but put the average 1.5 passengers in it and it matches the New York MTA and beats any train line in the USA by a longshot. It also beats the London Tube (250 wh/p-m in a 2004 report and many others around the world -- though European and wealthy-Asian systems do tend to do much better than US systems.

The calculation of optimal vehicle size will get an article of its own, but it is clear that that size is much less than the typical transit vehicle size today, at least on most lines. (It will vary based on line and load.)

The surprising efficiency of the 1-2 person vehicle

Today, in the USA, very small vehicles play a limited role. (In many countries, scooters and motorcycles are the most popular type of transportation, overwhelmingly when you add other solo types of transport like bicycles and walking.)

While electric sedans today use 250-300 wh/mile, lightweight single person vehicles can be much more efficient in energy, and also in road space. Single person urban vehicles -- effectively robotic electric enclosed tricycles -- have been built well below 100 wh/mile. The popular platform scooters used by companies like Lime and Bird use under 20wh/mile. 100 wh/mile surpasses the efficiency of any transit line in the world other than a pure rush-hour only system with lightweight vehicles, and no transit can compete with the platform scooters. Such vehicles are also very small on the road, and one can pack many of them in the space of a bus without the level of traffic disruption caused by the large bus. Solo vehicles offer non-stop direct travel on the optimal route, which can be highly efficient. They can also serve to offer "feed" and "fan-out" for vans, so that nobody travels away from their desired route, and transfers are instant and easy.

Private right-of-way

Transit's answer to the negatives of sharing is to allocate private right-of-way to transit lines, the most extreme example of which is the underground subway tunnel. Private ROW is very expensive, of course, but offers a fast and reliable path, free of congestion. There is an argument, in fact, that without private ROW, public willingness to ride transit drops precipitously.

With the arrival of the robocar and other technologies, private ROW and shared travel can be now seen as more independent factors. There is no reason that robocars and robotic vans can't be given use of private ROW, either old or new. We see a first taste of this in the use of "managed lanes" (Or HOT lanes) in some cities.

Existing private ROW, mostly for trains and some BRT systems, makes very inefficient use of the ROW. Trains almost always stop right on the track, blocking it, and thus must have a very long headway. A gap of 3 to 5 minutes, even at rush hour, is common, though it is possible to do better. Road vehicles tend to run with 2 second headways and it's possible to run robotic convoys on sub-second headways full-time since vehicles do not block the road in stations.

More to come

To understand where transit might be going, the first hurdle is to shed the old preconceptions based on the 19th and early 20th century technologies currently used in transit. In the future we'll examine more about the optimum vehicle size and the trade-offs for people of all classes of income and questions of comfort and time that go beyond efficiency, because in richer countries, people care more about the quality of their ride than the cost.