My dream is to have some sort of laser that would puncture the idiot's tire. Barring that, I'd like one that etched "butt head" on the rear panel of their vehicle. It wouldn't get him out of my way like the punctured tire would, but he'd know he'd pissed someone off by dawdling in the left lane.
I think we've shared this fantasy telepathically or something!
"Hmm... How can I mount a CO2 LASER in the engine compartment? I'll need some lenses and a mirror mounted on some servos under the control of a microprocessor..."