I suffer from the red mist sometimes as well, I am aware of it though and try to calm down.
Along with the feeling of power and safety we also get used to our cars as a private space so when someone threatens that private space we get overly protective of it and become far angrier than we would in other situations.
I think the road rage thing is magnified in this country though by the fact that Britain tends to be on the whole a pretty polite and helpful place* Even on the roads we let people out of junctions and wave thank you all the time. So when someone beeps at you you tend to get a lot more upset by it than you would in a lot of other countries where the car horn is used as a more frequent means of expression (everywhere in Europe south and east of Germany!)
*May not apply to London
