Part 15 operation on 87.9 here in the DC metro area is very rare to non-existent. That's probably because there's
WYPF, a class B1 FM, on 88.1 in Frederick, Maryland. There's also an annoying FrankenFM on 87.75 in DC that is probably operating at higher power than it's allowed, and it always QRMs 87.9 here.
I find Part 15 transmitters here all over the lower FM band, most usually in the range 88.7 to 91.7. The ones I've heard are almost always someone's personal jukebox or a relay of a Sirius XM channel in a car. There is also some crappy hands-free cell fone gear that transmits one side of a cell phone conversation on FM via a Part 15 transmitter. I doubt that the people who buy these things know that their conversations are transmitted in the clear to DXers and people who scan the bands regularly while they're driving.
Back when Howard Stern first started on the satellite, it was common to drive down I-270 during the morning rush hour and pick up Stern's show on several different frequencies in the lower FM band. I recall that there was a particular satellite-to-FM set that was later found to be operating at power levels exceeding Part 15 limits, and because of that, the manufacturer got spanked by the FCC. Around that time there were also a number of FCC complaints from soccer moms who were not pleased when they encountered Stern's uncensored content on their car radios.