Happy sounds like a very nice girl! I know my BC would freak out in that situation, crazy kids scare him ^_^ they scare me too.

Even though it's nice that she "knows" not to go into the road, you have no idea how many BC's who have been trained to "know" not to chase cars or run in the road have been hit and killed by them. Not only BC's, every breed out there. No matter what, to me, there could always be a moment that the most well-trained dog cannot physically resist their own instinct. And, to me, nothing is worth risking my dog's life. . . especially just the unpleasantry of having my dog on a leash and finding fenced-in areas to exercise him. Honestly, BC's are the last breed of dog to be trusted around cars and not to run into streets, ESPECIALLY when there's a ball involved! You should really consider doing what is truly best for Happy and leashing her. I don't want to annoy you, but I do say this to everyone that I see walking their dogs off-leash near roads. When I was 6 years old, I remember seeing TWO off-leash GSD's, jogging with their owner, run across the street (after a cat, or something) and both were hit by the car in front of us. I will never forget the horror of that (the owner was crying), and so many people I've met who have had a similar experience and hate themselves for it. Just IMHO