I would assume the acceleration pattern of being in a vehicle is somewhat distinct.
I'd be curious what happens when you are in a vehicle that drives completely steady (no turns/acceleration/deceleration) - would it eventually turn off? I guess you wouldn't get motion sick in that case either way.
I've used it on trains a bit and it does turn off when the train is going straight ahead. You might be going 200km/h, but if the speed isn't changing and you're always going forward you don't really feel any force, and neither does the phone, so the dots don't need to be there any more.
I'd be curious what happens when you are in a vehicle that drives completely steady (no turns/acceleration/deceleration) - would it eventually turn off? I guess you wouldn't get motion sick in that case either way.