Unless I am super exhausted and my body gives in and I end up just passing out when traveling, I do find it difficult to fall asleep on public transport. I find it easy as anything to fall asleep in say a family member's car but on public transport, it's a whole different story.
I can generally only sleep while sitting still unless I'm exhausted beyond belief. I've been exhausted enough that I fell asleep in the car a few times, but generally, it's hard to sleep in moving vehicles.
If I can sit comfortably, am not cold and I can set an alarm, sure. I used to have a commute that involved an hour's train in the morning - I'd settle in to the corner, put on some restful music to drown out the natterers, set an alarm and wake up in time for the trip across the rail bridge to get the next one.
I don't miss out on sitting by the bus window whenever I'm travelling. The moment we live, I'm sleeping off immediately especially when it's a far distance journey.
It's only when I'm not travelling far, then I won't sleep on the bus so the driver doesn't take me past my stop.