Most of the trainers and people I work/go to class with say they wouldn't trust a dog under 2 years old off leash period. Time spent training with increasing distraction and maturity both have something to do with it. It takes months in my opinion of testing with increasing distractions to determine/teach what will send your dog over the edge and what you can do to recall. Having some set up will help if you can get a friend to walk a dog friend by or act as a distraction. The more experience and training on different scenarios the better your knowledge of what your dog will bolt after and how to stop it. How about training with a long line so there is no obv leash and using body blocks to create boundaries.
If you don't concentrate on the concrete as the barrier it will probably be easier, since as you say you have a concrete drive which would be okay to walk on. I would try a tether on the long line and put a bungee on it or use a harness to prevent a hard neck jerk. Work on your stays and walk into the street, squat down, jump, make noise and reward for not coming over the line. Be close enough to body block any attempts to cross your boundary. Practicing just sitting in the front yard and watching things go by and getting and maintaining focus is your biggest tool. If you can distract and regain your dog's attention and cue him to leave things or come to you you'll get an idea of how he will react to certain stimuli and how to head it off. Is there a curb at the street? Maybe you could shape not stepping off the curb??
Do some work on a go to spot too which can be the middle of the yard or some other location so that you will have something for him to DO when the urge to go run comes up. Some dog's I've seen that were solid at the edge of the yard were actually taught to crawl up to the edge of the curb and not let their toes cross the edge of the grass.
Don't feel like you failed. You are doing things great but just need to up the ante with distractions and have some time training. This is a pretty evolved and complicated behavior to imprint and it needs lots of repetition with lots of different situations.
I like to hear that most trainers would not trust a dog under two off leash. Good to know!!! My pup is 14 months and is no where near being ready to be off leash. He will come to me in the dog park but only when he is dead tired and starts to pay a little attention to me. Then I call him, but don't say COME since I have been taught not to say come unless you are absolutely sure they will COME. So I say puppy puppy or his name and so he looks at me and tell him he is a good boy, but he has no time for a click and a treat---there are more dogs to chase!!! At some point he pays more attention and and then I click and he comes for a treat. We do that a few times and then I put his leash on him.
I also appreciate the tip about a harness when training on a long lead. That makes a ton of sense since he has choked himself once or twice running to the end.