Music Thing has a post about a DIY Road-Cone amplifier. While the creator of the project admits it doesn’t sound quite good, it is very loud. I think this might spark some ideas, perhaps a set of road cones that yell at drivers if they’re going to fast or ones that help teach driver’s ed students how to parallel park by telling you how far away you are and saying “ouch” if you hit them. Maybe not. Link.
DIY Road-Cone amplifier