How do they get them into the park? Carry them from the front door of the house? I think you are overstating the risk somewhat. Far more chance of being hit by a car en route to the park I wouldh ave thought.
Well, I don't know about you, but I supervise mine closely whilst walking, and hold their hands when crossing the road.
The same shouldn't really be necessary when letting them go and have a run around in a wide open playing field, but unfortunately that very quickly loses it's appeal when you're constantly having to watch where they're walking/running (to be fair, it's not just dog ****, there are glass bottles etc. to look out for too, but at least they're slightly more visible!)