Often people feel morally obliged to avoid doing something for the common good.  But as soon you charge a fee people will assume that that fee reflects the social cost and maybe start doing it a lot more.<p><a href="http:&#x2F;&#x2F;freakonomics.com&#x2F;2013&#x2F;10&#x2F;23&#x2F;what-makes-people-do-what-they-do&#x2F;" rel="nofollow">http:&#x2F;&#x2F;freakonomics.com&#x2F;2013&#x2F;10&#x2F;23&#x2F;what-makes-people-do-what...</a><p>Either don&#x27;t charge money for something and use suasion to get people to use too much or charge enough that you&#x27;re happy that they use as much as they can afford.