Motorists are incompetent.Cyclists don't take any notice of the red light.
Yesterday, I was waiting with several cars at temporary traffic lights. After nearly 10 minutes the light for us to proceed went green, but then a cyclist at the other end decided that as the single lane was clear, he would go against the red light and cycle casually along at about 10mph, while we waited with the green light until the cyclist had cleared off the lane. This meant that about 6 or 7 cars would miss the chance to get past the traffic lights and have to wait another 10 minutes.
I saw one crash once.