I agree that any playground near a busy road ideally should have at least some kind of fencing/barrier on the road's side. Alse where we live not all playgrounds are not (properly) fenced in either, some near pretty busy roads, and some have a barrier that would prevent children from just running into the road from anywhere on the playground, but the barrier is still low enough for a child to intentionally climb over it. So I basically try to avoid the play areas too close to a busy road, or the high and/or for my child less safe playground structures, and some I feel comforyable to go to with both of my children, but some not at all. When it's the schools playground, however, it's just inavoidable.
Since it doesnt look as if it would be fenced in anytime soon and your son wil continue in this school (at least for now?), I think it is really very important that his aide is shadowing him at ALL times. Imo, that doesn't mean just standing near the playground and following him with his eyes, but actually physically following your child from the ground and always being there in time when your son comes down one of the slides/steps.
That's what I do with my active risky behaviour child, there just is this NEED for a supervisor to always stand near the structure close to where he is on the structure to be able to follow him and to interact with him when needed (slow him down, if possible), and sometimes to find or 'capture' him when he comes down. He is 4 now and I am still doing this, be it a little less intensely than a few months ago, but I never was a parent who would (be able to) sit on a bench and watch my child(ren) from a distance. The rare times I tried this, I mostly regretted
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Maybe it could help having older children 'track' or guide your child on the playground, but I know that with my child this most likely would result in him feeling being chased and getting into a game and a 'panic' resulting in running on the structure and more reckless/careless behaviour, which is the opposite of what he needs :-). that's also what happerns if he's on the playground with his brother. It's just much more easier when I'm there one on one with him and no other children around :-).
And yes, some children just 'run away' without any obvious reason, but IF you could find out a reason(s) behind this behaviour it could be a way to address the need to run or maybe to help avoid this happening (too often), ıykwım?
So maybe it could be beneficial to have a (recess?) activity for him once a day which involves running: a game involving running, or have him run a certain distance (sports), and this in a safe area not so close near the road if it's the action he needs.
If I would be in your shoes I know I would not be comfortable either if I couldn't be sure the aide would actually shadow my child as a hawk at all times (especially at this playground!) and that when this aide is not available/having a break, definitely someone taking over aide's function for that time period.
I think you could praise yourself 'lucky' though to have a one on one aide available for your son. (I do not think such even exists in the country I live in nor the one I grew up in.) But I did follow some of your threads, so I know it's not the only school issue you've been dealing with lately :-(.
We were in the car tonight going to dinner and I look back and he's chewing on the buttons on his rain jacket. I asked why and he responded "because I'm hungry". Uhhhh..... okay. I offered him a corn chip (I had a couple left over from my lunch) and he refused, said he wanted the button 
: He puts a ton of stuff in his mouth too--almost worse than Mia, and the kids call her "Mia the Dyson". 
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