Three days per week, I childmind three 2yr olds and a baby. When out walking (with a double or triple buggy), any walking children will be on reins. We stop at all the roads, we are learning how to cross them appropriately. I don't hold the reins so tight that the child feels restrained.
They are not my children, and I need to be sure that they won't run into the road (they really aren't old enough to be left running beside a main road). If I have two children walking and am pushing a buggy, no matter how much "road respect" they may have, I cannot run in different directions if they choose to run away, or let go of one child (or leave the buggy) to get the other child. If we are in a safe area (nowhere near a road), they are allowed to walk without reins.