Do you let your kids play outside alone?

We let Ethan have the run of the property. It's fully fenced. Has big gates at the front that I shut when he is playing (dont padlock) Even if they are open, there is a "line" that he is not allowed to cross without an adult. We are on a fairly busy road, but if he is out the front, so am I. I usually sit on the patio or do some weeding. We also have gates that break the section up. They are on either side of the house. This means that Ethan can go out the back and play on this trampoline, swings, sandpit etc without me worrying.. I'll always leave doors/windows open so I can hear what is going on. But he seems to enjoy himself more when I am out there with him!!!