Port Orleans Riverside has (or used to have) bus waiting areas without chains or other guides to form a queue with. So whoever is closest to the door when the bus comes to a stop boards first. There has been no way to determine who was there first and bus drivers may or may not summarily board wheelchair guests first.
If there were an orderly queue, if someone in a wheelchair gets to the door and it is not practical to board him due to lack of appropriate spaces, guests further behind are not supposed to go around the wheelchair and board. So the correct procedure is for the bus to cease loading at that moment and depart.
The latter action can result in somewhat inefficient service. This writer considers that as a reasonable alternative, the bus in question could continue loading but another vehicle is produced within nine minutes (less than half the usual time interval between buses).
That is not really quite how it works.
All of the Moderate resort bus stops have open areas for bus boarding, without chains or other things to form a queue. That part is true, but each bus stop has a painted rectangle with a wheelchair symbol inside of it. Guests using wheelchairs or other mobility devices wait in or near that rectangle.
We travel to WDW a lot, are always traveling with a wheelchair and catch the bus at various places. I have never experienced the driver at resorts not loading the guest with a wheelchair first
unless that guest was behind people and was not at all visible to the driver.
If a bus stop has buses for multiple locations stopping at it, I advise people to do what we do - get out near the painted rectangle where you will be seen. As the bus is coming up to the stop, make it really obvious whether or not you want that bus. That would include shaking the head in an exaggerated way, nodding the head and pointing at the wheelchair. Doing that, we have no problems with the driver knowing we need to load.
If there are a lot of people, we might also have one of our group go near where the front door of he bus will be when it is stopped - they then say, "there's a wheelchair to load" - we've even had strangers speak up for us (usually the bus has the front door open because it is
unloading, not loading.
I've even seen the bus driver get off the bus before letting anyone on just go check if he/she sees a person in a wheelchair who is not paying attention.
There is no way that a bus 'on demand' could be produced to come less than half the time a bus would normally come. If the bus is full, many drivers will call another bus, but it has to come from somewhere, so can't be produced on demand.
This is a link to the bus page of the PortOrleans.org site mamabunny posted a link to.
http://www.portorleans.org/buses.php