For now, however, the map stands as a "here-be-dragons" document, warning where you can forget about getting any serious use out of your smartphone. Since the map doesn't cover 2G access, you could still hope to eke out a plain-text browsing session, place a voice call or send a text message in some of those blacked-out areas - but you shouldn't bank on it.
Counties highlighted in gray, meanwhile, offer generally consistent coverage and those in white don't have any 3G gaps - as determined by American Roamer's surveying. (In other words, this is not the map to look for confirmation that you're not the only person cursing at your unusable phone in the neighborhood grocery store.) Park the cursor over a county to see estimates of how many unserved census tracts, road miles and people the study found there.
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I tried to check the accuracy of the FCC's data against the coverage maps of the four nationwide wireless carriers - AT&T, Sprint, T-Mobile and Verizon Wireless. The black no-coverage areas of the FCC map matched up with the white zero-signal expanses of the carriers' cartography in the two regions I inspected in detail, the mid-Atlantic area surrounding Washington, D.C., and the Four Corners of Arizona, Colorado, New Mexico and Utah. (If you spot a mismatch, please let me know.) Nor did I see any surprising gaps; for the most part, fewer warm bodies and more mountains or deserts meant less coverage.