Turnout and First Past the Post

After the 2001 election, I plotted the turnout of voters against the majority.
http://www.greenhealth.org.uk/Democracy.htm

Turnout dropped by 10% when the majority moved from 5000 to 15000. In other words, not all voters are stupid. Some realise that their vote will have no influence on the outcome, and stay at home in safe seats. Therefore, if politicians really cared about voter participation, they would change to proportional representation, where more votes have an effect on the outcome. PR is also intrinsically more democratic, since the will of th epeople is more accurately represented.

The Electoral Commission know about this, but do not care.