At low speeds, an accelerometer determines the position of the lights by reading the pull of gravity.

CNN: Light-up wheels turn your bike into a 'Tron Cycle'

Later in Brooklyn, a squad car hit its siren and lights as he tried to pull a U-turn on Flatbush Avenue to get back to the Brooklyn Bridge.

WSJ: How to Run Up a $100K Cab Fare? Exit British Pub, Turn Left in Mumbai

We all see it on the roads, drivers who speed past only for you to pull up behind them at the next set of lights.

BBC: Traffic