Behaviours include but are not limited to - excessive and indiscriminate MP3 player use. Standing in a carriage doorway and not stepping off or moving aside. Barging onto a carriage as others are exiting. Stopping dead at the bottom of an escalator. Carrying extraordinary amounts of luggage in peak periods. Travelling in huge rustling packs, their vile neon coloured cagoules shedding static like a thunderstorm. Throwing themselves in front of trains in peak time. Bastards the lot of them.
Sufferers of IPS can be encountered on any mass transit system but the London Underground (see Hell) is particularly prone to the depradations of IPS sufferers.