Why Does The Traffic All Of A Sudden Come To A Halt On The Motorway? Where Does It Begin?

The traffic on a motorway can be compared to sand in an hour glass or in an egg timer.
If the traffic is slower than average on a small section of the motorway, it becomes the hour glass neck or a restriction point. The traffic approaching the neck is still going at an average speed. The slower speed at the neck becomes a partial blockage. More cars are approaching the 'neck' than leaving it at the other side. Gradually the cars approaching the restriction pile up and like the sand in an hour glass upper section become stationary. Traffic jam usually begins when there is no way a vehicle can move forward due to some obstruction.