I occasionally let my girls off leash to run inthe park, IF and only IF there is NOONE else there. I am not afraid they will be in a mood and snap at someone or something, but that they will get distracted and foreget to listen. I am constantly scanning to make sure there is no one entering the park and as soon as I see someone coming, I call the girls to me and put their leashes back on. I also call them to come back every minute or so and if they do not come almost right away, that is the end of their freedom on that trip.

I can totally understnd your frustration with someone having their dog off leash and not paying attention to it. That is a good way to get their dog injured or worse (or someone else's dog)