Our 4 year old black lab started off running last autumn whenever we were distracted when walking him and our terrier. He was off about half a mile away, we had to fetch him back in the car, there was a bitch there, but she wasn't on heat. After 2 episodes of this running off in a week, I had him castrated, (ie don't you mess about with me mate !) since then he's only run off once and otherwise is quite biddable and obedient. We've had 4 (entire) male dogs and this is the only one who's ever kept legging it.
The problem is that although the dog is chipped and has a tag on his collar and therefore could be returned to us he might cause a traffic accident and get injured/killed himself. I wouldn't want that to happen, hence the castration which does seem to have helped.