The biggest flaw in the "shared mixed use of roadways" is the auto-centric design of some roads.

Here in So Cal, where cars have long been king, it is quite common to find every road outside of a residential area a 50+MPH road. Those are high speed multi-laned roads. Sharing is just not on the minds of cell phone addled drivers. While a stripe is not perfect, those addled drivers do tend to stay between the lines.

Recently I started working in northern San Diego County. At least in this area, the bike lanes along the 50MPH roads are almost as wide as the rest of the lanes.