It’s definitely not accurate. I can show screenshots of over a dozen places with no service at all on AT&T, T-Mobile, or Verizon where this map claims coverage (and it’s supposedly filtered to be areas that have useable data, not just “coverage”).
As of a couple months ago, the T-Mobile coverage maps have been very accurate for both 4G and low/mid band 5G in my areas, and from what I've seen from others on the subreddit.
I can tell you it's lies in my area on AT&T. Extremely anecdotal though of course.
> The coverage map was created using data submitted voluntarily by the four mobile carriers using certain standardized propagation model assumptions or parameters that were established by the FCC as part of the Broadband Data Collection.
> Please note: The map depicts the coverage a customer can expect to receive when outdoors and stationary. It is not meant to reflect where service is available when a user is indoors or in a moving vehicle.
> Because the coverage map is based on propagation modeling, a user’s actual, on-the-ground experience may vary due to factors such as the end-user device used to connect to the network, cell site capacity, and terrain.