I travel the northeast (Acela corridor) by car and I can attest this map for voice coverage is not accurate to what you are actually capable of using.

Ironically, there is an area marked with coverage from all major providers that locally is of great contention because there is no cell service even for EMT, and ATT wants to put an antenna on the mountain over looking an untouched lake.

The locals would be quite happy to see there is no need for a giant ugly antenna to toxify their natural odyssey as they (according to the map) already have great coverage...