I know where my cats are at every second of the day. I couldn't handle the stress if I didn't know that they were always safe.
My whole back yard is cat proofed, so they can't escape. I live in the country, so my yard is pretty big and it's full of big trees and other gardens. The younger two (just about to turn one year old) spend the majority of the day outdoors, either chasing each other up trees and tearing around like morons, or they're sleeping on the chairs on the deck. The older two have a sniff and then a snooze on the deck furniture, then they spend the rest of the day inside, sleeping. They are NEVER allowed out after dark. I also bring them all inside if I am leaving the house - they happily come when called, because they know they get treats.
Weirdly, although there are a variety of places in the yard where they could go to the toilet, all four of them run inside to use the litter boxes and then race back outside.
Although the chances of them being hit by a car if they did get out are slim, I honestly believe that it's 100% on me to keep them safe and so, I would never let them roam. I've owned many cats in my life and not one of them has been allowed to roam unsupervised (I went through a very stupid stage of walking my cats on leads years ago - wasn't fun!).
I'm also very concerned about protecting the local wildlife. We have so many unique and endangered species in Australia, it's just good pet ownership to not allow your cats and dogs outside unsupervised.
I love my animals so much and I just couldn't bear it if I lost one due to something that was entirely preventable. I honestly can't understand why people think allowing animals to roam unsupervised is good for them. It's not.