ACS Commuter Data Visualizations at http://bigbytes.mobyus.com/commute.aspx
You can see a visualization of tract to tract commuting data for any county in the nation. I will just use this as an excuse to microscopically increment my own tech skills these days and figure out how to make an animated gif easily. Using the tool check out the daily commuting patterns for Butler County, PA, and in particular the big mass of folks who come down to work in Pittsburgh every day. Let's see if this works:
If you want to see some previous analysis of commuting patterns specific to the City of Pittsburgh, I have that, albeit in a far more static format here. What I saw there is reinforced by the graphic of commuting for Allegheny County. Check out what appears to be the largest green data point which looks like it moved down the Allegheny River. I am pretty sure that is singularly the commuting of residents of Fox Chapel to Downtown Pittsburgh daily.