From here:

Quote Originally Posted by sideswipes brother View Post
What about baseball bats? Thats what i have incase i ever get into a road rage incident. (them being the aggressors of course)
The law only allows you to use reasonable force in self defence. So unless the aggressor is carrying something that can be used as a weapon, you're not allowed to use that bat. e.g. If a person comes up to in a threatening manner, but they appear unarmed, you're not justified to use your bat -- if you do, either your attacker or the police can charge you for assault, even though you didn't start the fight.

Even if the other person presented a weapon (e.g. a tyre iron) and you use your baseball bat in defence, the police might ask you why you had a baseball bat in your car. Some martial artists recommend that the best 'weapon' to carry around are everyday objects that would seem reasonable to carry around regularly. A cricket bat or a baseball bat is an odd thing to carry around unless you're going to/from training... a better object may be a golf umbrella.

I keep a golf umbrella in my car -- not to use a potential weapon, but just in case it rains! And even if it's a blazing hot sunny day, we know that the weather forecast isn't always right. I actually have one umbrella in my car and one at work (because unless it's raining when I get to work, I usually don't take my umbrella out of the car, so I have a spare umbrella next to my desk in case it rains after I've left my car and I'm at work (cos it's a fair hike from my desk to the car park -- all uncovered outdoors).