Canada used to recommend 1 car-length for every 10 miles per hour. Along with metrification, that was changed to 2 seconds, but it's been set at 3 seconds for a long time.

I've yet to drive in traffic where even 1.5 seconds is manageable. More space than that and some slips into the gap, even if that leaves something like a loaded tractor-trailer hanging a second off their rear bumper.