Eventually, the upgrades could do away entirely with the red, yellow ... we go if it's green," said Hajbabaie, whose team used model cars small enough to hold. "But if the white light is active ...

A 1917 patent by William Ghiglieri also had two lights — red and green. But where was the yellow light? The fundamental problem with the red/green system is that of inertia. Cars can’t stop ...