Have a look at http://opensignal.com/ http://opensignal.com/ - the map is built by data submitted from their app on phones. Unfortunately they do not show area

Have a look at http://opensignal.com/ http://opensignal.com/ - the map is built by data submitted from their app on phones. Unfortunately they do not show areas where the phone reports no signal. In my area (Santa Cruz, CA) tmo are weak. Heck I barely get coverage in my house, and walking along West Cliff drive has sections with no coverage. Verizon is strong. However I very rarely actually use the phone as a phone so I mostly don't care.