Most interesting.
This seems right: "they had quoted using up to 4 transmitters per mile depending on the particular circumstances of a given area."
People can be very inventive.
Reading up on TIS/HAR stations can prove very interesting to a part15 broadcaster. Even though the greater majority of them employ the higher power (10w) transmitters, they still are closely related to our outdoor part15 systems, same installation and location issues, similuar grounding concerns, they must accept interference and can not cause interference.. etc.
Here;s one I recently reading from New Jersey Institute of Technology (1999) which goes into a little more detail of trials and errors involved concerning everthing from pros and cons of different methods of supplying transmitters with up to date content, different manufactures, turn-key system advantages, interferences, solar power install underground battery banks getting dammaged, and other things that might not readily come to mind. Titled: "ABSTRACT: HIGHWAY ADVISORY RADIO IN THE STATE OF NEW JERSEY by Thomas Mark Nemeth"
There's many more such documentation readily available.. a google search turns up lots of the same type stuff from different states, old and new..
above images from another interesting TIS/HAR report (only one of many such documentations to found): technical report standard page - Louisiana Transportation Research ...