Just had an add-on thought.
If Tesla had agreements with the private landowners that operate the SCs to have a 'holding area', they could use non-SC stall parking stalls as a place where a Tesla could be 'summoned' to by the car once it's finished charging and if the owner hasn't returned in time.
So, if car A has finished charging, and cars B, C and D are using the remaining stalls (4 total stalls) nothing happens. When car E arrives, if car A detects no driver in the driver seat, it automatically leaves the stall and proceeds to a designated "free-to-park" parking stall _outside_ of the SC area. Car E plugs in and is happy. Obviously car A owner gets the alert and knows where the car is.
All that has to happen is for Tesla to work with each of the SC site owners for a few hours to find a suitable place for overflow (if any) and pay any fees. These could be marked in the car as a SC station that has queuing available.
The same can work in a queuing system, where Tesla has a designated waiting area and the owners can drop their car off and get charged in a first-come, first-served order, but at least the owners don't have to stick around.
Being that it's private property, I don't think Summoning should be too big of a deal for regulatory issues, but I know it can be a grey area.
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Even if the supercharger is seldom used, you should not park there! As of now, you cannot know that there is not someone waiting to charge unless you are there.
I would like to see the charger status board that we were able to see in Hawthorne for awhile available in the cars. Also show cars that are charging, and those that are plugged in and not charging. Then be able to nicely 'ping' the owners of the ones that are finished charging via the app that there are people waiting for their spot.
I think you're splitting hairs. I think we all agree that certain SC manners are required. I'm just saying that I'm not cutting my romantic dessert short just to move my car if there's NO ONE else that needs the spot to charge. It's a Tesla software engineer team's job to find that solution that works best for everyone. They've done a good job so far, but they're a victim of the success of the system.