Hi,
My heart is just racing FOR you! My son is 17 months and loves to play the "run-away-from-mommy game". He most often does it in stores, but I know the day is coming that he will want to play it near a busy street.
We are starting to practice a new game called, "When-mommy-says-stop-you-stop-". I have lots of experience training guide dogs and pet dogs plus I was a preschool teacher before having my son. In the dog world and kid world, it is essential that you teach what you want to be learned in a non-threatening environment. Your son will not learn from you shouting and running, but instead, if you play "red light, green light" or "stop light, go light", STOP will mean a lot more in an emergency.
With dogs, in order to have them "come" when commanded, you have to practice it when it is not essential that they do so immediately. You do a few practice runs with a leash (no, I am not suggesting you use a leash on your son!!!) and give HUGE amounts of praise when they do it with or without leash. It may take a few days or weeks, but eventually, well-trained dogs do obey. I would imagine toddlers are the same - when we practice it with them and give them LOTS of praise, it will make it more desirable to do in the heat of the moment or at least make them think twice about continuing to run.
Good luck. I would imagine it is very normal to feel uber-protective after a close call like that. Go to a fenced in park or one that is away from the streets from now. Stand at the gate - in fact, make a point of leaning against it to build your trust in it.