Just from working in a municipal government with Rights-of-Way (ROW) and Easements. I would say that the vast majority of telecom infrastructure is in public ROWs and easements. My gut feeling is 90%.
Figuring how much exactly in on easements and ROWs, even on a small scale, is a huge a problem. Some places have hired someone to record the easements and ROW into a GIS system, some places, haven't. Some places have great maps of ROW, some don't. Until the state or federal government says you must, local governments won't. If money doesn't come to help, it still won't happen.