Earlier quoted context omitted.
It actually does seem to account for ferry routes. I plugged a location in Seattle in and the surface it draws juts across the water to roughly where the ferries dock. Maybe there just isn't a data source for where you're at yet?
The more I am seeing examples, and looking at the terrain, it seems like the problem is that it is only acting on the endpoints, and assuming that the map should simply extend from starting point to ending point, regardless of what is between them. But in the truth, it is quite possible to have an endpoint that cannot be accessed in a straight line because you must circle around a mountain, etc. These maps would need…
There's a similar blob going from Seattle to Preston, which appears to contain some impossible routes where you would get off I-90 and then circle around via highway.
As long as you stay out of the mountains and don't cross any water it seems pretty accurate as a first approximation.