I have mostly written about 3 and 4 wheeled Robocars, even when the vehicles are narrow and light. Having 3 or 4 wheels of course means stability when stopped or slow, but I have also been concerned that even riding a 2 wheeled vehicle like a motorcycle requires a lot of rider participation. It is necessary to lean into turns. It's disconcerting being the stoker on a tandem bicycle or the passenger on a motorcycle, compared to being a car passenger. You certainly don't imagine yourself reading a book in such situations.

On the other hand 3/4 wheeled vehicles have their disadvantages. They must have a wide enough wheelbase to be stable because they can't easiliy lean. In addition, for full stability you want to keep their center of gravity as low as you can. The extra width means a lot more drag, unless you have a design like the Aptera Motors entrant in the Progressive 100mpg X-prize, which puts the wheels out to the sides.

I recently met Chris Tacklind, who has a design-stage startup called Twill Tech. They have not produced a vehicle yet, but their concepts are quite interesting. Their planned vehicle, the Twill, has two wheels but uses computer control to allow it to stay stable when stopped. It does this by slight motions of the wheels, the same way that pro cyclists will do a track stand. They believe they can make a 2 wheeled electric motorcycle that can use this technique to stay stable when stopped, though it would need to extend extra legs when parked.

This is intended to be an enclosed vehicle, both for rider comfort and lower drag. The seat is very different from a motorcycle seat, in that you do not sit astride the vehicle, but in a chair similar to a spacecraft's zero-G chair.

In addition, the vehicle is designed to have the rear wheel on a lever arm so that it can stand almost upright when stopped and then slope down low, with the rider reclined, at higher speeds. The reclined position is necessary for decent drag numbers at speed -- the upright human creates a lot of the drag in a bicycle or motorcycle. However, the upright position when slow or stopped allows for much easier entry and exit of the vehicle. As everybody knows, really low cars are harder to get in and out of. Twill is not the first company to propose a vehicle which rises and lowers. For example the MIT CityCar plan proposes this so the vehicles can stack for parking. Even without stacking, such designs can park in a much smaller space. The Twill is not planned to be a Robocar at present. A human still drives it, though more as a car than a motorcycle, with computer-based systems providing stability. The vehicle however still will lean in turns, and the seated human will naturally lean with it. Done correctly, the passenger seat will always be tilted so that the force of modest speed turns is always net vertical to the passenger. Here the goal is to make turns more comfortable and less worthy of attention. It is also possible in this design to adjust the tilt of the passenger seat for acceleration and deceleration to make it closer to net vertical for the passenger. Still, at any speed, passenger attention may be needed to avoid motion sickness on turns -- this is still to be tested.

Two wheeled vehicles, however, can certainly pack much more tightly on roads than even narrow 3 wheeled ones. They can lane-split even on today's roads, and new special roads (including elevated tracks) can be made much more cheaply for them.

Another interesting advantage of the upright mode is that it would be possible for such a vehicle, particularly in robocar mode, to enter buildings and even elevators, at least during off-peak periods. One could easily see 3 or 4 in a typical elevator, allowing one's vehicle to go all the way to your floor. (Even if your robocar could not fit in the elevator when you arrived, it could come up later when the elevator is empty if its normal storage place is intended to be on the higher floor.) As electric vehicles, they could enter buildings. I can imagine some sort of system to clean the wheels with blasts of water and air on the way in, or we could also have elevator lobbies and elevators dedicated to the robocars that do bring in some dirt from the street.

You may be reminded of some of the robocar concepts found in the movie Minority Report. At first I thought it was quite silly that the robocars in this movie were able to climb vertical walls so easily, but one of the interesting applications was the way an apartment building worked. The car's magical ability to climb the side of the building meant it took you to your apartment and parked itself inside. One of the reasons that is interesting is that the major limiting factor today on building height is not structural, but rather the inability to make a practical elevator system. Due to the need for express elevators to support high floors, many tall buildings give up a huge amount of their real estate on lower floors to elevator shafts. Very tall buildings like the now-destroyed World Trade Center solved this by having elevator lobbies on upper floors and requiring a change of elevator. I have no idea how the cars climbed the walls (and I suspect the movie designers may not have any either) but it would be quite useful in tall building design. Of course anything that allowed elevators to operate more than one elevator per shaft, or even to pass stopped elevators, would also provide solutions to these problems.

Vehicles like the Twill, as light electric cars, would indeed be very efficient. Probably 5 to 8 times more efficient than today's cars and transit, even more if they are not intended for highway use as the Twill is.

The Twill web site is not particularly heavy with details on the main page, you will find most in their entries for futurist contests. There are other interesting projects aimed at light urban vehicles, many of which have application in the robocar world.