I keep a wheel brace, a can of Finilec (tyre repair/inflator), a gadget for removing wheel trims, a fire extinguisher.... why would I want to carry anything for self defence?
I have never been threatened in my car, but years ago on Blackpool prom, a flatbacked Transit joined the traffic from a side road somewhat ambitiously as I passed, and I had to swerve to avoid him running into the side of me. He then followed me to my destination and started a scene - apparently because my pillion passenger had shown him a hand gesture - akin to shaking a bottle of pop!! I remarked that the dents and scratches on his well used vehicle must have been the result of contact with drivers who were not so quick at reacting to his style of driving as myself, and invited him to call the police and make a complaint about my pillion - at which point he backed down and stormed off with wheels spinning!