Oh I can't wait to hear the responses to this question...
Some kids are just runners I guess. My son, who will be 2 next week, does the exact same thing. When we are in our front yard he makes a beeline into the street or the neighbors yards - he just basically takes off. If we are in any public place, same story. He is oblivious to calls to stop, come back, etc. I know age is a factor with lack of impulse control so I try to be aware of that, but man it is frustrating, not to mention scary.
We have instituted the time-out process - which had absolutely no effect I might add - until we started talking to him beforehand about what would happen (time out/leave) if he ran away from mommy or daddy or didn't listen when told to stop. It is SLOWLY starting to work. Now, as he is running to the edge of our yard or road and told to stop, he will either come running back into our yard (not towards me necessarily), or turn around and with a smirk and cross the line. Of course he is testing us in the latter case so he is promptly put in time out. When we are at the park, I tell him repeatedly before we actually get out of the car that if he runs away from us when told to stop we will leave. And then I do it. Unfortunately we have had to leave several places early, but I know it's important for us to do what we say we will do.
That's all I have tried so far, but if others can provide addtional stategies I too would love to hear them.
K.