I see this canard posted here quite a bit, that we shouldn't ever add road capacity because "OMG, someone will use it!!".

The #1 thing that creates more traffic is MORE PEOPLE in the area.  The problem is rapid population growth, which we've had here in spades for decades.  Even with our overcrowded roads that have failed to keep up.

Does adding more capacity to a particular road mean some people will shift over and start using it?  Of course.  Duh.  Traffic is like water.  It generally flows in the path of least resistance.  More capacity on one road doesn't just help out traffic on that one road, it helps out traffic on alternate routes in the area as well that were previously being crowded.  

Do good roads with plenty of capacity induce more total traffic than crappy crowded roads?  At the margin, some, yes.  But this crazy suggestion that traffic will be WORSE with more capacity than less capacity is just that -- crazy talk.