Traffic light

Traffic lights were signalling devices positioned to control competing flows of traffic. In River Song's World, where all of Earth's history occurred at once, a Roman soldier on a horse was seen waiting for traffic lights to turn green as cars went by. (TV: The Wedding of River Song)

On arriving in Pete's World, Mickey Smith, who was acquainted with the idea of parallel universes that diverge from the normal world, suggested blue traffic lights as a difference. (TV: Rise of the Cybermen)