Over the past few years I've some exaggerated claims about range with power levels of 20 watts or less. Some limiting factors can include antenna height, terrain and building penetration. While is might be possible to detect a signal with a sensitive car radio, building penetration would be nil at distances greater than a couple of miles.
The below link from RECNET shows the coverage area of WLSL-LP. If you select the Topo Map and zoom in, you can see the relationship between terrain and the radiation pattern. Note that in the FRINGE range there is no building penetration.
https://fccdata.org/?lang=en&facid=193453&appid=1692222&i=2
The problem with FM range claims is that range always depends on a number of factors, which are usually glossed over.
As you state and show, terrain (or geography) and obstructions (such as buildings) play a large factor. These are different for every installation.
The equipment used to receive the signal is also a huge, and mostly limiting factor. That includes, but is not limited to, the sensitivity of the receiver, its selectivity (i.e., the ability to reject adjacent signal interference), the location, type and size of antenna, etc.
Finally, the quality of the signal is important, and plays a role in the description of range. What might be a perfectly acceptable signal to some might be a static-y, filled with dropouts, signal to others. I suspect that programming plays a part here as well - you're more willing to accept a lesser quality signal if you're listening to talk radio, than if you're listening to, say, classical music.
Doesn't the FCC define a listenable signal for licensed stations, at least in the city, as 1 mv/meter field strength? That would certainly take into consideration the wild card factors such as terrain and obstructions, and it's a much larger field strength than Part 15 FM is, even at 3 meters from the antenna. Part 15 AM (at least from 15.219) at least would have this level of field strength for some distance (but not a mile, which is generally used as the range reference for a good installation).
Yes with regard to predicted signal strength. The useable (building penetration) is 3 to 5 miles from the transmitter site for a LPFM.
City Grade would be the closest contour nearest the antenna:
70 dBuV or about 3.2 mV
The next is the protected contour of 60 dBuV or 1 mV
A Ditto Here and a Ditto There
Yes this: "The equipment used to receive the signal is also a huge, and mostly limiting factor."
It's a sad state of affairs that after all these years there are continuous poor radios being vended by big names.
During these part 15 years in my life I've gathered a disappointing collection of useless FM radios. They drift outside of the bounds of their AFC & they completely ignore lower powered signals.
Fortunately there are the exceptional radios like TECSON PL-310, the most sensitive and stable FM receiver I've ever had, which goes to show that it can be done.
Then there's "...the quality of the signal is important".
In this location the quality of all signals is smeared by the over-abundance of signals... way too many 100 kW stations mostly doing the same formats turning the receive antennas into hot-sticks over-flowing with RF energy and messing with the radio front-end.
And I'll add... crappy programming! After tuning for half-an hour to check out all the stations it always turns out there's nothing to listen to... just common junk, making the internet the place to find variety.
I've had it up to here.