My theory is that drivers expect the others on the road to be capable of driving when, in fact, they aren't. If you drive like everyone else on the road is a complete n00b or a joyrider or something else equally as bad then you learn to be more careful as you are watching out for all the other idiots. The other day I was driving to Uni in the outer lane a motorway, when knob in a Beemer came flying down the slip road just ahead of me like his crotch was on fire. He pretty much didn't bother waiting for there to ample space to exit the slip road and came out between two cars on the inner lane, and straight across into the outer lane right in front of me in one movement. I had saw him fly down the slip lane and start to force himself between the two inner-lane cars, and slowed down, expecting him to go right out in front of me. He did. And if I hadn't have reacted before it happened he would have ended up in my passenger seat.