Let's start with our common understanding of what a vehicle is, and consider its implications.
From Wikipedia:
http://en.wikipedia.org/wiki/Vehicle
Vehicles are non-living means of transport. They are most often man-made (e.g. bicycles, cars, motorcycles, trains, ships, and aircraft), although some other means of transport which are not made by man can also be called vehicles; examples include icebergs and floating tree trunks.
Vehicles may be propelled by animals, for instance, a chariot or an ox-cart. However, animals on their own, though used as a means of transport, are not called vehicles. This includes humans carrying another human, for example a child or a disabled person.
Vehicles that do not travel on land are often called crafts, such as watercraft, sailcraft, aircraft, hovercraft and spacecraft
Most land vehicles have wheels.
Movement without the help of a vehicle or an animal is called locomotion. The word vehicle itself comes from the Latin vehiculum.
Inherent in our understanding of a land "vehicle" is that it is a carrying device that uses something other than legs to support the passenger or payload. Since tracks are usually too damaging to be allowed on public ways, the normal support is wheels.
Although it is possible to build a land vehicle that can pivot all of its wheels to allow travel in any direction, this is usually impractical; nearly all vehicles have at least one wheel that does not pivot. This gives nearly all vehicles a turning radius longer than their length, and makes it possible to move only in the direction perpendicular to the fixed axle. This is a serious kinematic limitation compared to that of legged animals, e.g. pedestrians.
Many vehicles commonly achieve momentum in travel that, due to dynamic effects, requires a much larger turning radius than predicted by kinematic analysis alone, and requires significant time and distance for starting and stopping.
Lastly, for both kinematic and dynamic reasons, wheeled vehicles have difficulty negotiating discontinuities in height in the traveled way.
One can consider these kinematic and dynamic constraints as significantly reduced maneuverability compared to legged travel. This cost is often worthwhile for the benefit of greater speed capability, reduced power requirements at a given speed, and greater payload capacity compared to legged travel. However, the reduced maneuverability has important implications for how one should negotiate the traveled way, particularly in regards to other vehicle traffic.
So, vehicular cycling starts with an understanding that bicycles have maneuverability constraints similar to those of other vehicles; those traffic negotiation rules that were developed with the consequences of limited vehicle maneuverability in mind are likely to have some significant importance to bicycle travel in traffic. Similarly, the design of public ways for bicycle travel should incorporate consideration of these maneuverability constraints.
-Steve Goodridge