Many of the more interesting consequences of a robotic taxi "mobility on demand" service is the ability to open up all sorts of new areas of car design. When you are just summoning a vehicle for one trip, you can be sent a vehicle that is well matched to that trip. Today we almost all drive in 5 passenger sedans or larger, whether we are alone, with a single passenger or in a group. Many always travel in an SUV or Minivan on trips that have no need of that.

The ability to use small, light vehicles means the ability to make transportation much more efficient. While electric cars are a good start (at least in places without coal-based electricity) the reality is today's electric cars are still sedans and in fact are heavy due to their batteries. As such they use 250 to 350 watt-hours/mile. That's good, but not great. At the national grid average, 300 wh/mile is around 3000 BTUs/mile or the equivalent of 37mpg. Good, and cleaner if from natural gas, but we can do a lot more.

Half-width vehicles have another benefit -- they don't take up much room on the road, or in parking/waiting. Two half-width vehicles that find one another on the road can pair up to take only one lane space. A road that's heavy with half-width vehicles (as many are in the developing world) can handle a lot more traffic. Rich folks don't tend to buy these vehicles, but they would accept one as a taxi if they are alone. Indeed, a half-width face-to-face vehicle should be very nice for 2 people.

The problem with half-width vehicles (about 1.5m or 4.5 feet if you're going to fit two in a 12' lane using robotic precision) is that a narrow stance just isn't that stable, not at decent speeds. You like a wide stance to corner. One answer to that is the ability to bank, which two-wheeled vehicles do well, but which requires special independent suspension to do with 3 or 4 wheels. 2 wheels is great for some purposes, but 3 and 4 have a better grip on the road, particularly if a wet or slippery patch is encountered.

There are quite a number of 3 and 4 wheelers with independently adjustable wheels made. Consider the recent concept I-road by Toyota which exemplifies this well. There are however a number of vehicles that are not concepts, and this (rather long) Gizmag video provides a summary of a variety of real and concept vehicles in this space, as well as enclosed motorcycles and scooters, including the Nissan Landglider, the VW 1L, the Twizzy, the Tango, the Lumeno Smera and many others. Skip to about 13 minutes to see many of the 3-wheelers. Another vehicle I like is the Quadro -- Watch this video of the 4 wheel version. These vehicles are aimed more at the motorcycle market and are open, while I suspect the single person robocar will be an enclosed vehicle.

I also wrote earlier about efforts on two wheels, like the concept vehicle the Twill. Other recent efforts have included the gyro-stabilized Lit Motors C-1 which can be fully enclosed on two wheels because you don't have to stick your legs out.

I suspect the 4 wheeled bankable vehicles are the ideal solution, and the technology is surprisingly far along. Many companies prefer to make 3 wheeled vehicles because those currently get classed as motorcycles and require far less work to meet regulations. These exemptions are reportedly ending soon, and so the effort can shift to 4 wheels which should have the most stability.

The ability to bank is important not just to stay stable with a narrow stance. Banking also means you can tilt the passenger to make turns more comfortable in that the force vector will be mostly up and down, rather than side to side. In a turn it feels more like getting heavy and light rather than being shifted. Some people, however, will have trouble with motion sickness if they are not themselves looking out the window and feeling part of the banking move. Being able to tilt forward and back can have value so that starts and stops also produce more up and down force vectors rather than forward and back. While this is not yet demonstrated, it may be possible to make vehicles which provide minimal discomfort to many passengers when doing things like turns, stops and the roundabout. Roundabouts seem like a great idea for robocars in many ways, since you don't need to have stop signs or lights, and robocars should be able to insert themselves into gaps in traffic with precision and confidence. Frequent roundabouts, however, would be disconcerting with all the turning and speed changes, to the point that many would prefer just a straight road with timed traffic lights, so that a clever car that knows the timing never hits a red.

Another entry in the narrow vehicle field that got a lot of attention is the autonomous driving Hitachi Ropits. The Ropits -- here is a video -- is a narrow vehicle with small wheels, and is able to be autonomous because it is super-slow -- it only goes 3.7mph -- you can keep up to it with a brisk walk -- and is meant to go on sidewalks and pedestrian lanes, more of a mobility for the aged than a robocar. However, it is a new entry in the autonomous vehicle pantheon from a new player.

The big question that remains about these vehicles is crash safety. As motorcycles they are not receiving the same sort of testing. In a world that is mostly robocars, one could argue that you don't need the same levels of crash safety, but we aren't there yet. All is not lost, however. Recently I sat in a prototype of the Edison2 Very Light Car. The VLC is a 4-seater with a narrow body but a wide stance, for handling. This vehicle has been crash tested with good results, and it could be made with independent suspension and banking and a narrower stance if the market wanted that.

Small vehicles, just 4.5 feet wide and 10-12 feet long can make a huge difference. First of all, they are inherently (except the Tango) going to be light, and light is the most important thing in making them efficient. But they will also take up less space on the road, able to go 2 to a lane (or even lane split in some places.) They will also take up much less space parking. The combination of their small size (about 1/3 of a typical car) and their ability to pack close together "valet style" as robocars means you will be able to fit 4 or 5 of them in the same amount of parking lot area that today fits a single car in a non-valet lot. As noted, while many robocars will not be parking at all because they will be taxis that head off to get their next fare, those that do wish to park will be able to do it at vastly greater densities than we have today, and the consequences of that are big.

There are a few other options for increased stability with normally narrow stance. These might include:

Low center of gravity -- this is what the Tango does, filling the very bottom with lead-acid batteries. Passengers might sit lower -- some vehicle designs involve lowering after the passenger gets in.

Variable stance: a possible ability to widen the stance with an extendable axle so the vehicle takes a whole lane when in places that need that cornering ability and stability.

Extra wheel: The ability to temporarily deploy an extra wheel (probably not a drive wheel) to one side or both to temporarily increase stability. This wheel might take all the weight on that side, or balance with the others. Vehicles side-by-side could even coordinate to still fit in a lane but that sounds risky.

Just go slow: Narrow stance vehicles might just be used in lower speed urban routes, and take corners fairly slow.

Gyroscopes, under robotic control.

It's important to consider that the risk of instability in a narrow vehicle is mostly one for human drivers, who are used to wide stances and may make errors on the physics. A robocar, with full knowledge of the vehicle's characteristics and the shape of the road simply won't try any turn that would tip it, and it won't pick routes that have turns that would require the vehicle go so slowly as to impede traffic. Knowledge of road traction can complete this sort of analysis.