The sun was rising in the city of Irvine, California, while cars began their morning commute on Interstate 405. A driver traveling well above the speed limit looked to his right to change lanes, failing to notice that traffic had slowed in front of him. He slammed on the brakes, but realized it was too late. He braced for impact.

It was supposed to be just another beautiful, quiet, normal autumn morning in Atlanta, Georgia. Two public service vehicles — a school bus and a waste management truck — were both traveling southwest along Metropolitan Parkway. At the same time, Audrey Carson was traveling east along Casplan Street on her way to...