That's why city buses have 3 or 4 double doors and there's ticket machines inside (and drivers don't sell tickets). The time to board rarely goes over 15 second

That's why city buses have 3 or 4 double doors and there's ticket machines inside (and drivers don't sell tickets). The time to board rarely goes over 15 seconds. Compare: https://www.lubus.info/images/stories/taborbus/5122-57.jpg https://www.lubus.info/images/stories/taborbus/5122-57.jpg vs https://www.chicagobus.org/system/photos/250/large/DSC00925.jpg https://www.chicagobus.org/system/photos/250/large/DSC00925.... That's double the boarding time at every stop right there. The schedule is also designed in such a way that the bus is usually ~1 minute ahead of time and can wait for the proper time to depart from each bus stop - zeroing the randomness on each stop. If it gets too delayed on one part of the route it can catch up on next few bus stops. On intercity routes there's fewer bus stops so usually there's just 1 door and the driver sells the tickets.