Sometimes, walking along a pavement, you come across a couple of people who are clearly walking together, or more accurately ambling, but rather than being close together they strategically place themselves so that you can't easily walk past them on either side - so you have to slow down and look for an opportunity to get past: but so do all the other non-ambling walkers heading in same direction, so before you know it you are colliding with either the two amblers or others trying to get past.
There should be a walking test, like a driving test, and also a 'how to push a supermarket trolley' test.