Clearly, all of this will have some effect on congestion and road capacity. Accidents themselves cause as much as a third of congestion (estimates vary a fair bit and depend whether you're talking about highways or city centres), even if there are no changes from different driving behavior. How much changes over all, though - how much more traffic can a highway hold? How much more quickly do you get to school in the morning if you drive at the same speed but don't have to stop at every stop sign just in case there's someone there? We'll find out.
Carpooling, of course, has been touted for decades as a way to use cars more efficiently. But it never took off because it suffered from an information problem: There was no way to coordinate rides on the fly, no way to know whether someone four blocks away was heading in the same direction as you, right this instant. Safer just to drive yourself, right? And this gave birth to a welter of personal choices that seemed perfectly reasonable individually, but that together created a massive environmental and urban land use problem—with many of us heading off to work in the same direction and with cars that contained, statistically, only people each.