School bus stop arm

Every school bus has a small red stop sign on a hinged arm on its left side. When the bus stops to pick up or drop off children, the arm swings out and the red lights flash.

All 50 states require drivers to stop for a bus with its arm out on a two-lane road. Rules for wider roads with a median vary by state.

Many districts now put cameras on the arm to record drivers who pass. That is how Austin counted its Waymo violations.