We have one for our DS who is another fearless runner. We called it his "buddy" rather harness or leash. I do hold his hand as well though, so it is a back up system in case he lets go of my hand and makes a dash for it. I found it a godsend when I was heavily pregnant as he could easily outmanouver me!

I do think it is important to teach children safety and listening etc, so that is why it is a back up. I will say "stop" and "hold mummy's hand" etc and try to teach road safety etc.