Most of us already know that it's legal to turn right on red unless otherwise indicated by signage. But there's an occasion when it's acceptable to turn left on red as well.
Back in my day as a young bean growing up in the farm fields of back-country Illinois, my Driver's Ed course taught that when you approach a four-way stop at the same time as another driver, you defer to the person who arrived there first and then work clockwise. If four people arrived at all four stops at the same time, you make eye contact with them to determine who will go first, and then proceed to follow the clockwise rule.