Interesting. Has it occured to you that the real sniper might write just such a letter as a way of controlling the greatest number of people?
Even if the advice given is good, enough people would ignore it that he wouldn't run short of targets... but it would let him control many people he'd never get a chance to shoot at.
I think it's great advice. We live close to an interstate in the DC metro region, and I want my family to stay safe until the killers are caught.
They kid me about being paranoid but they listen to me.