I noticed this on some footage I took on the way into work today. Some of the more modern (or updated) London buses have LED rear/brake lights and indicators. I suspect the brightness of the rear lights is probably adjusted by pulse width modulating the drive to the LEDs. The frequency this is done at is near, or is some near multiple of the frame rate of my ATC2K, since you get a flashing effect on the footage which clearly isn't visible to the naked eye (or wasn't to me on this occasion). This short video shows the effect on a Bus just in front of me at the traffic lights on the north side of Chelsea bridge.