is this one, which Brian and I came up with while riding back from the UVA game last weekend. Route 29 has got a lot of stoplights, and at highway speeds it's tough to tell when you ought to stop for a yellow light and when you ought to keep going. The problem seems likely to come to a head as red light cameras are rolled out.

The image above expresses the basic idea. The yellow light now has multiple states, counting down from a full light to whatever minimally-thick donut-shape is safely visible (better to go from full-to-donut than full-to-point to prevent problems with depth perception at night). This is all possible since incandescent traffic lights are rapidly being replaced with LED lights. All it would take is the addition of a cheap timing circuit -- I don't know much about circuit design, but with a morning on the internet and an afternoon at Radio Shack I'm pretty sure I could whip one of these up.

I'll admit that this might seem kind of silly. The yellow light is already a transitional warning -- do we need warnings about its transition as well? Reductio ad absurdum, etc. But the yellow light interval isn't standardized -- this would give drivers an immediate, intuitive sense of how long a yellow will last. The traffic light system wasn't designed for modern highway speeds, and the cost to implement this would be minimal. Making sure folks know what the hell it means before they get on the road might take a little effort, but I bet some warning signs could handle the job.