Now that I've played with the app for a few days, it's clear that it has some general overlap with TM, but its overall purpose is different:
- Users get credit (10 points) for a highway if they have driven any part of it, driven past an intersection with it, driven over or under it, or driven within some radius of it
- Users get a bonus (1 point) for traveling on, or past, another part of a highway they've already received credit for (It's not entirely clear under what circumstances the bonus is awarded: it might be a certain distance away from the part previously encountered, or a part that's in a different county, or something else)
- Users are not credited for clinching a highway, beyond the 10 points and possible bonuses listed above
There is no way to tell how much of a highway has been travelled in the app. Crossing a highway at every intersection could earn a user the same number of points as driving its entire length.
So I'm not sure how Highway☆ might benefit from TM, or how TM might benefit from Highway☆, other than cross-promotion. Ultimately it might be nice to have a TM app, but this shouldn't be it, IMO, since it serves a different purpose which is excellent in its own right.