It could complement but not replace a general purpose system, and anyway the bigger problem is not navigation but avoiding other cars, pedestrians and obstacles

It could complement but not replace a general purpose system, and anyway the bigger problem is not navigation but avoiding other cars, pedestrians and obstacles on the road. Since your solution is based on sending someone to mark the roads, why not use the same someone to simply build a map of the road, Google Maps style with higher accuracy? I think there are a few startups doing just that now.