Many people tend to think that level 5 vehicular automation—a car that can truly drive itself under any conditions a human driver can—is just a step beyond level 4. However, levels 4 and 5 are a world of difference. Here’s what D&D* have to say about it (slightly paraphrased):
Subworlds, like the world of physics, the business world, and the [world of roads], make sense only against a background of common human concerns. They are local elaborations of the one commonsense world we all share. That is, subworlds are not related like isolable physical systems to larger systems they compose, but are rather, local elaborations of a whole, which they presuppose. If [restricted domains like an ideal highway] were subworlds, they would not have to be extended and combined to reach the everyday world, because the everyday world would have already been presupposed in programming each [restricted domain]…. [T]here is no way [such domains] can be combined and extended to arrive at the world of everyday life.
In other words, driverless vehicles in controlled environments (like a highway specifically constructed and maintained for driverless vehicles) are possible. Driverless vehicles in uncontrolled environments are not possible under current and foreseeable technology.
*Dreyfus & Dreyfus, Mind over machine, 1986, p.76.