Well, that isn't a straight-forward problem. Take Atlanta to Zurich, for example (a Delta flight I have been on). Conventionally, the plane flies up the East coast until it clears Nova Scotia, in part because that is approximately the great circle route, but also because pilot training 101 tells you that your flight plan must include nearest emergency landing point for every inch of your route.

So for super-sonic flight, that path might be modified to be off-shore enough to keep sonic boom over the ocean, but follow the East coast closely enough that an airport capable of accommodating the plane for an emergency landing is always within a reasonable distance. (Bonus points for being able to fuel up and take off again.) It isn't simply a problem in spherical geometry.

East to west US coast doesn't fly over unpopulated for substantial periods. Despite popular belief, people still live in the middle of the US.

The only places in between with enough space to get up to supersonic and subsequently drop out in time for population are a few chunks of Nevada, Utah, New Mexico, and Colorado. Look at a night time satellite picture of the US to see how infeasible flying over most of the eastern US is.

if it's only the transition _to_ a supersonic regime that generates a shockwave, then it wouldn't be out of character for carriers to get a variance allowing for "acceleration zones" which could be within 20 minutes flight time of most hub airports. If your plane leaves JFK and goes out over the Atlantic to go supersonic before heading to Seattle it would still be quicker...

It's not. The shock wave is present as long as the plane is in super-sonic flight.

I grew up in fly-over land, and military training flights based out of the Dakotas used to go overhead; we would get the occasional boom.

(Also, once in a while doing my farm-kid chores driving a noisy piece of machinery across a hay field I would get buzzed by a jet sneaking up from behind me flying at a very low altitude. It was always startling because I never heard them coming over the noise of the tractor. I'm sure I made a pretty good surrogate for an enemy tank...)