The study I referred to was not done by Arbitron.
I did the study with our in-house research division, SIP, back in the early 2000's for HBC to allow a better evaluation of stations they bought, particularly rimshots. We looked at 5 of the top 10 markets and 5 of those between 11 and 25 and 5 in the 26 to 75 rank. We tabulated the data on ZIP Code vs home and work (when known) location for millions of diaries of adults 18 and over.
We found that 80% of all fixed location listening was in the 70 dbu coverage area and 95% was in the 65 dbu area. Nearly none was outside the 65 dbu area, and those that were generally were very close to that contour or, per Langley-Rice, in an area that actually had better coverage.
Since there has been no significan change in receiver technology in the last 15 years or so, the results are
Sounds like very interesting research. Would love to see something like that done for today's Greenville. However, regarding statements to the effectiveness of 40dbu signals in reaching listeners, you must have discovered that passionate formats tend to perform better, even through signal challenges. WZWK has a passionate audience.
FM stations are protected out to their 60dbu contour. However, they are generally quiet, even down to below 40dbu. Where the issue of quieting becomes an issue is when stations suffer interference from co-channel and adjacent channels. Some stations do well out to their 30dbu. Some you can't hear beyond their 60.