Hmmmm ... I don't know about clever insults but ... when I am driving I enjoy dividing other drivers in to categories of 'twits' (who are simply naive idiots, unaware of their poor / dangerous / unsettling driving styles) and 'twerps', who are quite aware of what they are doing and ought to be banned from the road.

I find that involving myself in this simple categorisation makes my tolerance of other drivers a lot higher than it might otherwise be, and prevents me from using much stronger language.