Momentum (or kinetic energy) as modified by human reaction times, which in turn are a function of the road design and other local conditions. That implies signs

Momentum (or kinetic energy) as modified by human reaction times, which in turn are a function of the road design and other local conditions. That implies signs which are not only tailored to their location, but can automatically adjust based on weather and human activity patterns. Streetlights come the closest to approximating this, but I'm not aware of streetlights that react to weather. (Road design is a fascinating field; the intersection of physics, materials science, automobile engineering, and human psychology. It's possible to design a road to allow the average person to drive a modern midsize sedan in relative safety given good weather conditions, but you have to make the road a closed system (controlled access) and become zealous about maintaining it and the cars that use it. The Autobahn, in other words.)