Four police cars caused long motorway delays when they crashed into each other on a training exercise.
Two lanes of the motorway were still closed at lunchtime
The pile-up happened at 1045 BST on the M61 northbound near Chorley, Lancashire, caused tailbacks across motorways north of Manchester.
Three experienced road traffic officers suffered whiplash while another was trapped in his vehicle with more serious whiplash injuries.
Four cars, three marked and one unmarked, were involved in the crash.
Two lanes of the motorway were still closed at lunchtime around junction nine
A Lancashire Police spokeswoman said: "As far as we are aware only police vehicles are involved, no members of the public."
An investigation has begun into the crash, which did not involve new vehicles.