One meter, pretty standard since I can remember in (many/some parts of) Europe and Canada iirc. This is common sense, who's going to leave less than that gap at speed?
On my commute home from the office, there's a short stretch (about half a city block) between where I exit from a low-traffic residential street onto a main thoroughfare and where the road widens from 2 lane to 4 lane + bikelane. On that stretch I get buzzed uncomfortably fast and close about once a week.