Here's a link to a Colorado Dept of Public Health study done in 1996:
This link was on the website which I listed in my first post.
Also, there's a good article on page 5 of the pdf located on the following website which I created for the anti-beltway group (CINQ) when it was fighting possible compromise by the City of Golden: http://www.homeingolden.com. Page down to page 5 after the PDF opens on your screen.
Full disclosure: I'm on the board of that anti-beltway group and remain a vigilant and outspoken opponent (unlike my fellow Realtors) of the toll road and beltway proposals.