There are legal restrictions on who can direct traffic, so standing there with a flag at a roadblock might be okay but that person can't actually direct anyone to do anything unless they've been certified for it. Also, people are assholes. If they think there's no consequences for just ignoring your roadblock, they will - I've manned roadblocks during storms and had people call me an asshole, drive around me, and go straight into 5' deep flood water. Same type of roadblock with a cop at the end of it doesn't usually see problems like that. If I had a construction site going that I really really didn't want people driving into, I'd prefer to have a cop doing traffic control over a flag-man any day.