Flow rate problems on the Kentmere Fell Race

The race got off to a steady start with all competitors moving off at a reasonable pace. However the back of the race that we could see came to a standstill very quickly. Maybe the road narrowed or maybe the first competitors hitting the slope to the church made them slow down. I think that the race must act as an incompressible fluid - or at least when the competitors are close together at the start. The race got me to look up Bernoulli's principle that I understood 30 years ago. Some work to be done to get the understanding back https://en.wikipedia.org/wiki/Bernoulli%27s_principle