Except that quickly becomes inaccurate if the bus driver decides to re-open the doors for an incoming passenger, or due to traffic conditions, opens the bus doors further from the stop than the beacon reaches. Or skips a stop due to road conditions.
Notice how we're now adding more complexity now anyway though...
This feels like a dangerous hack that will come back to bite you in any number of unexpected scenarios that probably occur frequently during an average bus driver's day.