Problem is; certain laws that get in the way. The laws of physics.
With an analog FM booster, there will always be a certain amount of overlap. This isn't just a transmission challenge, but a reception one. Every receiver has a different capture ratio based on so many factors. That, and the combination of receiver, receiver location, antenna length, (in the case of cars) vehicle size, other man made or natural reflective surfaces around the receiver, I could go on and on, all effect the reception.
We studied something similar in the early days of DTV. The idea was that rather than the traditional high powered transmitter sitting atop some hill, why not build a 'cellular' approach of installing smaller, lower power transmitters all over the market? But those studies were COFDM digital modulation UHF, not analog VHF FM Broadcast. It worked pretty well, but the receivers were ATSC receivers not used in a mobile environment.
