The growth of LTE networks continues to astonish even those of us who meekly predicted near-ubiquity as far back as 2009, US Telecom's Pat Brogan estimates LTE coverage at 99.5% of the population, based on company data reported to the FCC. Both Verizon and AT&T claim 98% coverage and T-Mobile almost as high. They have different unserved areas, so 99.5% amongst them is plausible.
The real figure may be as low as 95-96% because coverage data is notoriously unreliable, with companies having incentives to inflate claims. I remember AT&T telling the FCC a while back the FCC didn;t need to test wireless availability because the companies could provide accurate maps. I checked AT&T's map of Manhattan and saw it claimed as 100% covered. I live here, and at the time was hearing numerous complaints of deadspots.
It's much better now. I've never failed to find a signal on T-Mobile or Verizon in Manhattan in the last two years.
The maps are made based on computer models of cells and terrain. They aren't perfect. When I worked on an LTE project in Vermont in 2011, both Alcatel & Ericsson sent in top analysts. They were frank about the limits: accurate on average, but not in particular details.
In the diagram, US Telecom's 99% estimate is to the left, then Verizon's 98%. It's nearly impossible to see the yellow representing those not covered. The uncovered bar remains minimal even at 96% served, which assumes serious lying by the reporting telcos.