Penis size notwithstanding it occurred to me that at least one reason for this aggression is related to the kind of signals that you can and cannot make to other drivers. It is relatively easy to thank someone, for example, for letting you out of a junction or into a motorway lane. From the front this can be done by flashing the headlights, or from the rear by either using the hazard warning lights (something I first encountered in Cape Town) or by doing that funny thing where you quickly flick the indicators left then right then left again. Either way, the meaning is clear "Thanks", "Cheers", "You're a good 'un".
You can send negative signals too. By hanging onto the your light flasher for a split second longer than for the "thank you" signal you can say "get out of my way". Horns can be blasted and fingers held aloft to say "Twat" and its many variants.
But the hardest word for your car to say is sorry. Weird that, particularly in the UK where we seem to spend most of our time uttering those 5 letter as we negotiate our way through the social milieu.
So here's the big idea. A sorry light. This could simply be a light on the tail of the car (as it seems to me that for some reason that you nearly always want to say sorry to the person behind you, possibly as a result of cutting them up). I'm not sure what colour it would be but its function should be clear, flashing it sends an apology to the driver behind. You could even imagine subtle differences in flashing patterns evolving a quick flash could be done when you just nip in front of someone infringing their personal space but not requiring their touching their brakes. This would be a the kind of sorry that means only a little more than the "excuse me" that you mumble when having to push through a gap in a queue at the airport in order to stop your kids losing their fingers on the baggage carousel. More elaborate flashes could be used when the driver behind has to take evasive action: stamping on the brakes, for example, or switching lanes. More like the kind of sorry you would say having airily putting your cigarette out in someone's half-full can of coke. Which I did once. On a train to Hatfield, as it turns out.
Of course as Biologist Amotz Zahavi taught us, any signalling system is open to misuse. I used to know someone who'd ride his bike through a crowded pedestrian area in the centre of Manchester elbowing people out of the way and each time shouting a cheery "Sorry!" Here the signal is fake; he didn't mean sorry in the true sense, all he meant was "Don't hit me" and it worked! The Zahavian problem is that if more and more people use a signal in this deceptive sense then fewer people will pay attention to it and ultimately the signal is ignored. How many times have you seen hazard warning lights used to signal a hazard rather than being used to excuse the fact that someone has parked on the pavement or in front of a fire station or something.
Zahavi argues that one way to preserve signal honesty is that they should be costly to the sender. Thus the sorry light could give the driver a painful but non-lethal electric shock when making it, or maybe it could automatically text your name and address to a "mea culpa" list on the Internet. Or possibly instead of flashing a light, pushing the sorry button could flash up a photograph of the transgressor in a sexually compromising position on the rear of the car. Actually scratch that: there are already too many things for drivers to fiddle with as they drive without providing impetus for yet another.
But of course even these solutions could be exploited by masochists and exhibitionists, so maybe they should pay by being sent to Hatfield.
Sometimes a signal can cost too much.