Bad drivers come in two general categories. There are careless drivers and reckless drivers. These categories can be further broken down. Here are some examples:
Inexperienced Drivers : These are generally recently licensed teenagers, or people who for some reason have never really learned how to drive. They generally drive with a certain lack of confidence, and are somewhat easily distracted. They may have little appreciation for how important it is to keep eyes forward on the road, and may not check their blind spots (or use their turn signal) while changing lanes. If possible, it's best to pass them, otherwise keep plenty of distance to minimize getting into an accident in the event that they rear-end someone. Inexperienced drivers tend to get in rear-end and sideswipe collisions the most. I once had a carload of teenagers (being a teenager at the time myself) nearly run me off the road into a snowdrift by simply failing to check their blind spot and yield properly. About a month after I first got my license, I rear-ended someone myself, and quickly learned how important it is to "live in the moment" while driving; the present is all that matters, pay attention, because your life is literally at stake!
Elderly Drivers The stereotype that older people cannot drive has some basis in reality, but is not necessarily true in every case. There are some people who are way too old to be driving. Often they may use their turn signal, then fail to turn it off, but I've also had this happen with more median aged people as well. In every close call I've had with an elderly driver, it was a matter of them not checking their blind spot. Elderly drivers tend to drive at the speed limit or less; some drive ridiculously slow. If possible, pass them up, it should be easy. If not, don't harass them, they'll just go slower to piss you off.
Irrational Speed Demons : I say "irrational" because I myself speed, and think that there is a time and a place for it. In my home state, there is technically only one absolute speed limit -- 75MPH. Sure, there are speed limit signs everywhere proclaiming 25 in a residential, 45 on main streets, 55 on highways, etc. But technically, you have the right to argue in court that the conditions were safe for you to drive whatever speed you were going. I personally think it's foolish to speed on anything with a speed limit of 35 or less. If it's that low, it's usually because there are many people present, and people can show bad judgement and it's your responsibility not to kill them for it. Vehicular homicide is no fun.
Irrational Speed Demons don't understand that it's unnecessary to ride someone's ass. If the traffic in front of you is going 35, you won't be going any faster than 35, whether you are five inches or five blocks away from the car in front of you. It makes no difference! So why not stay a few car lengths away, especially at high speeds? I constantly see people on the highway driving so fast and so close that they could easily be killed if traffic were to halt for any reason...and it does, sometimes. Shit happens. Be prepared. Especially while driving in snow, or driving in rain. Don't be an aggressive driver when it won't get you anywhere, and don't even think about it in bad conditions. You'll only make everybody else nervous, and they'll probably just go slower.
And if you get tailgated by an Irrational Speed Demon, just go slower. Generally they will get the point. If they happen to be driving in an SUV with blinding headlights, flip your rearview mirror to cut it down to size. If they get more aggressive, there's no traffic in front of you, and it's safe enough to do so, pull to the shoulder and let them pass.
Confident Teenage Punk Drivers : Generally they have recently obtained a license, but are experienced enough to feel confident driving at high speeds. They are often Irrational Speed Demons, and may or may not be equipped with a car that is far too powerful to be trusted in their hands. Usually they are not as aggressive as stressed-out "I wanna go home" adult males driving oversized pick-up trucks, but they tend to blast annoying music loudly and may or may not be driving a car equipped with an aftermarket muffler or merely an exhaust tip designed to make their car sound as obnoxious as possible.
Your best bet when handling any case is to simply understand the concept of defensive driving. The best defensive driving strategy is simply a matter of keeping yourself aware of everything around you. Crazy, unexpected things can happen while driving, and everything can change in a fraction of a second. Use your mirrors and check your blind spot while changing lanes. Use your turn signal, and don't tailgate. Whether or not you believe in karma, the best way to drive is to drive cooperatively. Don't be antagonistic or oblivious. Be aware!