My take after reading this is a bit different than the others. I think it's absolutely insane that a three year old has to learn traffic rules to not be mauled to death by passing cars. What a fail of society.
A 3 year old shouldn't be in traffic unsupervised of course, but it can't hurt to help them understand what's going on around them. Even at age 3, kids are interested in that stuff. And eventually they do have to learn this, and by the time they're old enough to be in traffic unsupervised, they should already know it. So you start early.
Yes. But they shouldn't have to. It's just a testament to how car-infected society is, that you have to move around based on their demands. Why should a car be able to drive straight through my street, while I have to go out of my way to find a safe crossing? It should be the opposite.