I never intentionally trained for invisible boundaries for roads, but being consistent in the behaviors that I expect of my dogs during walks has led to it. I always expect my on-lead dogs stay off the road and to sit before crossing and now they offer the behaviors without my asking it of them. Even off-lead they plant their butts to the ground once they hit the curb and look back waiting for a release cue. They also won't go past a certain point after the mail box and I suppose that's from all the times I've taken them along to help bring in the mail.
With my older spaniel, he was offering these behaviors after 1 week of on-lead leash training (flexi lead) in high traffic areas. He's not the most biddable of dogs, but he loves his walks and was very motivated to keep going. If your own dog is stubborn, then you'll have to find a way to motivate him more. I use the flexi lead in training so that the dog can have more freedom, but I can still maintain control. It's a great step up from a 6" lead before moving on to a check cord, though I wouldn't recommend flexi leads for those dogs who don't already have good manners on a normal lead.
I'm doing the same with my Cocker pup, but I don't use treats to reinforce the behavior since continuing the walk after a nice sit-stay is reinforcement enough. She's still at the stage where we only use a normal lead. She's already rock solid in her invisible boundary training for the bathrooms in the house, which was done by watching her like a hawk and verbal corrections. It's nice not having to be so obsessive about people closing the bathroom door after they use it.
I would suggest you have clear visible boundaries for the dog, like the curb for roadways or flags in the yard. The flags can always be removed after you've proofed the training. As always, proof the behaviors on a check cord before off-lead training and proof behaviors while building up distance, distraction levels, and duration. I also would not move on to off-lead training unless the dog already has a solid recall and a solid emergency sit. With the older spaniel, I continue to proof the behavior by occasionally not saying a release cue even after I have crossed the road myself or throwing a ball over a road.
