I have this same problem....
We live on 12 acres, but when I let Victor out of the fenced yard, (we had to put one in just for him) he heads for the barn to visit the cats, and horses. But the barn is much closer to the highway, and he runs all around and will not come to me. Boxers can be stubborn and so intent and focused on what "they" are doing it is hard to get something out of their head. We have had to install a backyard fence for Victor, because I am afraid he would just run right out into the road (and it's equal to about 2 blocks away from our house). He will not come to me when he is running. But it is obvious to us that he knows what he is doing, and after we have caught him, he sits there fully expecting a mean, bad dog talk, and takes it with an air of, "I know, yeah, I get it, okay, okay, I know, and I am sorry...But....."
He is the funniest dog I have ever known. Also the smartest. So now he only goes outside the fence on the leash, or after dark. For some reason he stays close to the house in the dark. One good benefit is that on the lead he heels extremely well, instead of "walking me". I know other Boxers who don't leave their person's side. My first Boxer was that way. But not Victor. He is very social. I am hoping that as he matures that will change. At least for now, I just can't let him go free. Hey, I am open to suggestions, too. He comes very well otherwise, in the house, or yard. He knows all the basic commands, and shakes hands. He will be a year old in September. Meanwhile I just keep working with him until maybe the "come" will become more of a habit with him.