My Linus is 5.5 months old and he is in this phase. I have not heard it called a Vagrant phase before, however.
You are NOT alone. Here is one of many threads on this topic.
I've stepped up obedience work but he is still backsliding a little on some things like attacking the couch. He is also teething now. Last night and this morning were particularly bad. He's cutting a new molar tooth and, just like a human toddler, he is cranky from the teething pain and acting up.
I have pretty solid recall on Linus and he does well off leash but I only let him off the leash in very safe areas (e.g. with a fence or other barrier or really far from anything like a road). I politely suggest you reconsider letting him off so near a road. He's just shown you that he is not to be trusted. It would be a tragedy to see him run down.
Best of luck,
Linus van Pelt Haus Juris CGC. My Sweet Babboo, born March 17, 2014.
Madeline Ronan Heart (Maddie) Dec 1997 -- November 22, 2013 Rescue GSD, Sable