If you made every car computer controlled, using GPS, GLONASS or whatever (and God help us if any of these systems ever went down as it would effectively grind the entire country to a halt!) then a car could merge with traffic, or change lanes easily, as other cars would be made to stop. Cars would communicate with the others.
Ordinary motorists wouldn't automatically let a computer controlled vehicle in, so driverless cars would no doubt have to emergency stop in such situations, as well as going slow in the first place (to ensure the passengers aren't thrown around too much) and then working out how to merge with fast moving traffic - no doubt taking far longer than ordinary drivers.
Computer control COULD let cars go a lot faster and closer together, but there will probably be many regulations to stop this from actually happening. I actually expect we'd see cars driving slower, until such time that you built special roads free of uncontrollable risks, like pedestrians and animals. You'd probably be fencing these off, like motorways or indeed railway lines.
If roads were also kept clear of parked vehicles and other obstructions, this also begs the next question; where would all these vehicles stop/park when not in use?
It would mean a massive spend in infrastructure to allow for all of this to happen, while many motorists would be very resistant. Otherwise, driverless cars will NOT be simply allowed to drive as normal amongst everything/everyone else.
I expect that we'll see cars controlled by a driver, as now, but with a stack of technology to make it easier to drive - including cars able to communicate with each other and avoid collisions (but only ever stepping in to stop a collision, not actually drive the car). A lot of that technology exists right now, and can be bought on, say, the new Volvo V40 - but that can only stop itself, not another car (for now).