If all cars had them, I think this would be a good idea. The biggest problem is that all cars won't have them, and it would be ~30 years before it even became n

If all cars had them, I think this would be a good idea. The biggest problem is that all cars won't have them, and it would be ~30 years before it even became noticeably rare to encounter a vehicle without them. If you can't assume vehicles have front brake lights, the lack of an indicator has no direct meaning. If you see on one, great, that car is slowing down -- but without comprehensive deployment, you can't assume that no light means that the car isn't slowing.