A few reflectors aren’t going to go over budget more than the crap they do. You can pork barrel it since it won’t be that much more. Roads are often updated if there’s any sort of traffic on it by design since construction is a racket.
A few retroreflectors is not even scratching the surface of the issue. Most local governments don't even have accurate maps of their own road infrastructure, let alone correct infrastructure. They already can't afford to maintain what's already built, much less take on significant additional maintenance. Humans are able to cope with these issues most of the time, but for machines you'd end up needing to build nearly the same autonomous capabilities you'd require without smart roads.
Stop signs are notorious hard to detect. They could make them easier to detect.
I'm not sure where you heard that. Stop signs are one of the more highly recognizable signs in my experience. They can be obscured, rotated, be used in commercial signage or otherwise distorted, but these aren't things that would be helped by a different design.