Michael Tobis models the lawnmower problem:
If you have a typical American house, you have a typical lawn in front of it, a lawn that is in need of occasional trimming. Unless you contract out for lawn services, you almost certainly own a lawnmower too. Most likely it has a cheesy, loud, polluting little engine.
You only use this for a half hour every other week, or 1/336 of the time. OK, you don’t want people mowing lawns at night, so say 1/168 of the available daylight time. So you and your 167 nearest neighbors own 168 times too many lawnmowers. If you could coordinate your lawnmowing, you would need to spend 1/168 as much on a lawnmower.