I've seen this before but I always marvel at it when it happens. There is a domino effect when you serve people, even quietly. After Tyce and I shoveled some spots, my next door neighbor Raul did the same the next day. Another neighbor routinely snowblows about 6 or 7 houses worth of sidewalk, including mine. Another family shoveled out a spot and saved it but then thought better about it the next day and got rid of the spot savers.
And now, a week into heavy snowfall, only one person on our block has kept out any spot savers. We've chucked those milk crate markers twice (in Christian love) and then parked in another "unsaved" spot. We're trying to communicate that parking spots are for anyone. I think our communication is pretty effective.
There's a culture of serving emerging which is being shaped by... serving. This is a very powerful spiritual principle--serve before the culture values serving and watch people start to value and participate in serving in a deeper way. Now certainly this is not foolproof--some people will refuse to change even if everyone else is serving. But by and large, in every social sphere and group, I believe you can sow seeds of serving and watch others begin to sow them along with you bringing a harvest of good.