I like the idea of reins but I've never been able to use them with my lively nearly 4 year old. From one years old he hated anything that srapped him in and escaped from cot, pushchair and car seat. He was an exceptionally strong and determined wriggler and climber. I did try a wrist strap when he was 2 years but he just wouldn't accept it with me. It was a failing on my part - he did accept a wrist strap with his childminder at the time. But he definitely had lots of other routines with her that he wouldn't carry over at home.
My solution - and it's not one I'd recommend to anyone - is to give him endless piggybacks near roads. Or I hold onto his wrist, not his hand or hold onto his collar (he's happy with the latter even though he hated me holding onto a wrist strap).
We manage somehow but I have had some hairy moments.
Some parents like me might face HUGE and prolongued resitance from their children over reins, since children are all different. In that case, reins are going to seem an extreme last resort, possibly verging on the cruel. I don't think using them is cruel btw, just an impossibility in my case.
I just could not go through the trauma of seeing my son so upset, I'm sorry.
I am lucky in that I am physically capable of still carrying my son, especially giving him piggybacks. I don't have a younger child in a pushchair to worry about either. He loves chatting to me while he is being carried, he is safe and happy, so that's what I do. I am gradually phasing it out - he is getting past the toddler stage and is getting more road aware, but I don't expect I'll stop the piggbacks completely for at least another year. I did exactly the same with my oldest son too so I hope my back holds out!