Yes, I believe you are agreeing with me. Cars can do that on their own. no central computer needed.
No, if the cars' actions are not coordinated in some way, either by communication with one another or central control, they then know nothing about what other cars are going to do until they do it. With central control or communication, you could have several miles of cars on the freeway, all traveling at 200+ mph, separated by mere inches. One needs to get off in a mile, and the others all know it, so they gradually make room for that car to move over and get off. If there's a reason to slow down, they can all do so together. If each car only knows about the other cars actions when it actually happens, they can't coordinate their movements and they have to keep sufficient distance between them and go slow enough so that they can react appropriately to the actions of other cars.
Now, I suppose you could have the cars communicate with each other with brake lights, turn signals, and such, but then the cars will only know about the cars in their immediate vicinity and can't really coordinate their actions with more distant cars.