At the risk of sounding somewhat "cold"; to be honest, I've never understood why scooters get priority.
It can be frustrating to wait bus after bus while scooters simply ride up and get access to the very next bus with no wait.
OK. Posting in order to be informative only...
ECVs & wheelchairs load first and exit last because they need room to navigate them into proper position in order to safely load, unload and allow the driver to secure the
ECV appropriately for transport. It is difficult enough to back an ECV onto the narrow lift, then basically parallel park it in the designated area--doing so on an already loaded bus would be even more difficult (not to mention potentially unsafe).
Also, keep in mind that each bus with a working lift can only accomidate 2 ECVs/wheelchairs max. This means that if multiple handicapped persons (more than 2) are waiting for bus transportation, we wait longer... because we have no option but to wait for another bus (or two... or...). In the mean time, many if not all of the 'able bodied' guests can walk right onto the bus (even though it is unable to accomidate any further ECVs)--and once the regular seats have been filled, they also have the option of boarding a 'standing room only' bus if they so choose. The ECV users have no such choice--we sit and wait, sometimes for an hour or more, for a bus that can accomidate our needs.
As for family members boarding a bus with the ECV user...
How would all of you like to be told that you can not ride on the same bus as your son, daughter, wife, husband, etc.? Yet this is just what a lot of 'able bodied' guests are suggesting that we do--send our disabled family member off on one bus, to go wait at a drop off point somewhere for the rest of us to catch up with them later.
In all honesty, there is no 'advantage' to needing to use an ECV at WDW. For every one time when we actually may have gotten into an attraction slightly sooner, we waited longer than other guests who entered the line at the same time at nine or ten others (due to limits on the number of 'handicapped' guests permitted in an attraction at any given time, or due to having to wait for an accessible ride vehicle to make several rounds (there is often only one). And then there are the attractions we love... but which my family member could not experience, due to mobility issues that made it impossible to transfer safely. It was heart renching to have to tell him there just was no way for him to experience POC, for example.
