My 2 yr old plays unsupervised in backyard ( check when it goes silent) yard fully fenced and gate locked, quite rare she is the only kid in backyards though. Older 2, 8 and 11 play in the street with the 10 or so other kids and have since 4 & 6, with that many kids you find out pretty quickly if something has happenned. 2 yr old not allowed on street so she sits in front yard and sulks lol. I don't really like when one kid plays out front alone though...only when at least another child out there ....but usually street is full.
At our old house never, I think it does depend on how busy street is and how many other kids playing. As other kids do watch out and all other parents are popping their heads out to check.