The park we usually go to is great about this. The norm is that ALL kids drop off their sand toys in the sand box when they get there. EVERYONE plays with whichever toy they want to. The working it out and taking turns gets dealt with the same as the rest of the playground equipment... it has nothing to do with the person who owns it, and no adult really supervises the turn taking except to help a child work it out (ie, "why don't you ask him if you can use it after him?"). The only time you hear "that's mine" is when someone is getting ready to leave. (the exception is on Saturday mornings when a million families are there waiting for soccer games and it's a madhouse)
Kids do ask about using others' ride on toys which makes sense because different families have different rules about helmets etc. I wouldn't enjoy getting in the middle of policing turn taking though.
In general, it works well for me just to let kids work it out or point out when someone is waiting for something, but not try to police how long each kids turn can be. Just like the swings, or slide, or monkey bars... kids can use them as long as they want. Part of waiting your turn is... well... waiting :-/
HTH,
T.