Driving defensively is illegal and just as likely to cause accidents as driving aggressively.
You must mean something different from what I mean when you talk about driving defensively. I do that all the time, and I've been coached on it for many hours both in the classroom and on the road by serving police officers. Most of my time on the road is riding a motor bike, and on a motor bike you're very vulnerable. If a bike is involved in any way at all in almost any kind of incident, it doesn't usually matter who made the mistake. Generally speaking it's the bike rider that gets hurt. If I hadn't learned fairly early on to drive defensively, then by now I'd have been killed several times. Mostly by people driving cars, but occasionally a lorry and even the odd bus. Never a bike, which seems odd (because some of the worst drivers on the road are riding bikes) until you realize that it's just a numbers game. There are more cars on the road doing more miles than anything else, that's all. Everybody makes mistakes, but fortunately most of the time the mistakes aren't fatal. Most of the time, they don't even have any real consequences except perhaps a quick burst of adrenalin. I'll come to that later. But the more mistakes that are made, the greater are the chances that there will be consequences from one of them.
So to me, driving defensively just means reducing the chances that mistakes will happen and that when they do happen, the consequences are less serious than they might otherwise have been. For example when I'm driving I'm always reading the road, the traffic and the conditions so as to assess where the next incident is going to take place, and then trying to be somewhere else at the time. If there are bumper-to-bumper queues of traffic in lanes two and three on the motorway, then I'm probably in lane one.
I've been disappointed by some of the comments in this thread, which have seemed to me to betray a poor attitude to road safety. Driving on the roads is the most dangerous thing you can do in peacetime (well, apart from flying in the space shuttle which is in orbit for the last time as I write). Deliberately doing things to irritate other drivers is calculated to raise the temperature and increase the probability that a mistake will be made. Even if I'm not on a bike I don't want to be involved in anything like that. It only takes one mistake to ruin the rest of (or end) your life, or someone else's life.
I said I'd get back to adrenalin.
There is a well-known response in the human body when unexpected things happen. It's called "preparation for fight or flight". It is the result of the evolution of our species in a dangerous place. For millions of years, when something unexpected happened to us (and our ancestors) it usually meant that urgent action was required, simply to ensure our survival. Using its sensory inputs, the brain notices the unexpected thing, and we immediately get that sinking feeling in the pit of our stomach. That's what's known as an adrenalin rush. It's designed to allow us briefly to get the most from our muscles to protect ourselves from whatever danger has befallen us. The muscles perform better with the adrenalin boost, and we can ignore pain more easily. We can run away faster or fight the foe more robustly. When we use our muscles to do those things, the adrenalin is consumed, and that's the end of it.
Fast forward to the age of the mechanically propelled vehicle. Now, unfortunately, if we're driving, that's not the end of it. When we're driving, unexpected things happen. As I said, it's just about the most dangerous thing we can do in peacetime. Deep down at some primeval level, the brain knows that. When unexpected things happen, the brain pumps us up with adrenalin, just like it's been programmed to do by millions of years of evolution. But our muscles aren't doing anything. We're just sitting there. The muscles aren't burning up energy, they aren't burning up the adrenalin, and the result is that the blood stream fills up with an obnoxious soup of chemicals which should be being burned off by muscular exertion, but isn't. The more unexpected things happen, the more the soup in the blood builds up.
This build up of chemicals has undesirable side effects. It affects our brains. It is responsible for our disproportionate reactions to the things which happen on the road. In some people it is extreme, and the well-known 'red mist' is the very dangerous result. Whether you suffer from red mist or not, when you can recognize what's happening, and take precautions to avoid it, you will be a much safer driver. Taking precautions to avoid it just means knowing what's going to happen before it does. That just means giving yourself time to read what's going on around you, instead of piling into situations which might turn out to be recoverable and might not. Then there are no surprises, no adrenalin rushes, and you'll get to where you're going in a serene state of mind instead of getting there in a foul temper -- or perhaps not getting there at all.
Take care, it's a jungle out there.
