I get the point of the 'expected Passenger volume' system in career mode, but in sandbox mode it doesnt feel right as it hinders/limits the players route creation creativity if one wants to have 'very high' passenger volume on all created routes. I therefor think that route creation in sandbox mode should have an option to adjust the 'expected passenger volume' for any desired route.
I believe the "expected passenger volume" is fixed and only refers to the manual route design in career mode - in order to statistically serve passenger satisfaction.