Caution flags and collars for dogs.. what do you think

I was reading an article that suggested  the use of warning flags  on dogs leashes..  Do you think these are a good idea, or an invite for problems.    I think that if people read and honored the flags, they would work, but the people who really freak my dog out, aka small children, would ignore them and run right up to my dog anyway.  Why do people not teach their kids its NOT ok to try to hug dogs they do not know??   My dog is very shy so I would like to use these so that people will back off and let him approach them.