Does it make a difference if it's backyard vs front yard, busy street vs not busy (quiet neighborhood)...etc

My kids are 2 and almost 4 and I don't let them play outside by themselves. Our backyard is big and private but no fence and the driveway connects front to backyard. We live on a 25mph road but people fly through here and street aside there's no way. The kids across the street at 4 and 6 and ride their bikes up and down the sidewalk alone all the time. So I'm curious what age is ok for you guys for your kids to play outside alone, and under what circumstances.