The tutorial workflow has this component controlled by AITrafficLightManager, but you can also write your own scripts to control a light for a route.
Red light mesh, disabled for green and yellow.
Yellow light mesh, disabled for green and red.
Green light mesh, disabled for red and yellow.
Route to control, cars can't exit assigned route if light is red or yellow.
List of routes to be controlled, cars can't exit assigned route if light is red or yellow.
Cars can't exit route. Red mesh enabled, green/yellow disabled.
Cars can't exit route. Yellow mesh enabled, green/red disabled.
Cars can exit route. Green mesh enabled, red/yellow disabled.