At first I thought the "unmanned tunnels" description was just a way to avoid broadcast regulator scrutiny, but it does look like it's genuinely designed to be used underground as part of an emergency alert system. That led me to "leaky feeders", a type of broadcast antenna used in mines and tunnels. https://en.wikipedia.org/wiki/Leaky_feeder
I must be missing something ... Why broadcast speech over AM in an 'unmanned' tunnel ?? who's gonna hear/receive it ?? I wish the repo had some sort of use case summary or something... Edit: I'm also sorta puzzled by the choice of AM in any sort of 'alert' context...Do people still listen to/use AM radios?
Updated the README with a use case section - vehicles with AM radios transit during construction/maintenance. Leaky feeder distributes the signal. Thanks for the feedback :)