given that people generally don't spin their tires off the line from a stop sign, it's fairly safe. If it's obvious that two people are going at the same time then one that has advanced less far in that time will generally ease off the throttle and let the more advanced party continue.

there are accidents, of course, but i'm not sure of any 4 way traffic junction that's infallible.

and as others stated, right-of-way rules are observed atop the social norms.