In California, to different degrees in Southern and Northern California, people are generally fluid about lane changes and other traffic maneuvers, given that there are so many people trying to get around. If you attempted to rigidly apply some spacing and discipline to all the drivers, there would be no movement (see any of the articles written about the detrimental effect of metering lights at on-ramps). People simply flow around each other fluidly in an effort to keep forward momentum. This is not to say that people don't get their tempers up (road rage) from time to time.
In the Midwest, by contrast, there is much more rigidity. Fewer drivers, with many coming in to town from rural areas where there are very few other cars, causes people to behave in a more courteous manner, but also are much more territorial about 'their space'. This applies to lane changes, following behavior on roads, and so forth. When you attempt to react fluidly to a situation (change to the other side of the road in a parking lot to assist the person in the other car from moving out), you are met with hostility, as if you are impugning their driving ability somehow by your accommodation.
Here in London, the drivers practice advanced fluidity. They seem to move like water in and out of lanes (their own and others), create their own routes, and otherwise keep the flow moving at all times. There is no hostility expressed when the taxi drivers swerve in to oncoming traffic to avoid a bus, the other drivers simply move out of the way, as if this is completely normal and acceptable behavior (which it is).
I recall taking a bus ride through rural Bali in 1998, and the bus kept moving into the oncoming lane on a two-lane-road, when you could clearly see traffic headed our way. The oncoming traffic would simply move on to the shoulder of the road until we had passed, then regain their own lane. The first time this happened, I thought this may be some advanced form of Indonesian-auto-chicken, however by the end of the trip I was inured to the practice.
My suspicion is that this fluidity of movement is an extension of people living in such close proximity to each other for so long, as I have seen it in Milan, Rome, Paris, Madrid and elsewhere. Ironically, it's just the opposite in New York, where people are notoriously territorial in traffic (horns and fingers at all times). In Iowa, it is gradually evolving from farmers waving at each other from their trucks as they rarely pass each other, to insurance executive's luxury SUVs passing each other with great frequency. With this addition of drivers and traffic, you can see the 'rigid' rejecting the onsets of fluidity.
This causes me to wonder if there is some other pheremonal version of this for spacing of pedestrians and homes. It certainly seems to make rural people twitchy when confronted with the seething mass of New York or London.