[Kodak]

Heh, the horn seems like the most immediate response. My only thought is that drivers that are first car at a light may interpret a horn to mean "hurry - you're wasting my time!" - resulting in a quick acceleration...backwards. On the other hand, it may have the desired effect of bringing their attention to the actual problem.

I took this one step further and asked myself a related question.

Which would be better in this situation: backing up as far away from the car as possible - giving the driver more reaction time to stop (but also allowing them to be at a greater speed in the case of a collision), OR driving right up to their bumper - practically tapping it - to eliminate the chance of them speeding into you, but increasing the chance of a little bumper to bumper contact?