When I practice recall, I tell Missy to sit and wait, then release her with 'okay'. As she runs towards me I tell her "go on!" giving her permission to run ahead of me... then once I feel she's run far enough ahead I call her back with 'okay' and 'come!' As soon as she hears a car, unless I hear it first, she stares at the road... I tell her 'leave it' or I whistle and she used to stop... hopefully I can get this back. (when I hear the car before she does I call her back and she comes - equalling a JACKPOT!), or if she sees a car coming she stands and waits near the end of her 50' lead and then runs full speed at the side of the road barking (that's when I'm too late in calling her before she gets into chasing mode - oops!)! If I notice the car before she does I call her back and reward her for coming. I do try my best to keep it under control... where she'll run ahead and before she gets to the end of her 50' lead I call her and she comes most of the time. Usually about 95% of the time I keep it fun for her of course! If she doesn't come, I give her a 'bump, bump, bump' on her collar and when she returns to me, I send her off again and try another recall shortly after. Once we've practiced some obedience: recall, sit, down, wait, okay!, etc. then I bring out the toys and we play fetch as her reward... she'll get the ball but if a car comes and she notices it, she lets the ball go and goes to chase the car... I tell her 'leave it" and "get your ball" and that seems to help to refocus her on the game. Did I accidentally teach my dog to chase cars without realizing it?