The Democratic gains in the county are primarily due to the exurban migration of disproportionately conservative, Republican-voting residents to the neighboring counties of Collin, Denton and Rockwall As a result, those counties had been continuously more GOP leaning than Dallas, until diversification and continued movement of college educated whites away from the GOP has led to those counties being more competitive. The tremendous growth in these neighboring counties was part of a larger explosion in exurban growth throughout the nation over the last decade which coincided with the real estate bust in 2007. In North Texas, exurban growth was accelerated by transportation infrastructure expansion including the widening of U.S. 75 north and the extension of Dallas North Tollway in the mid 1990s, and the completion of the George Bush Turnpike after 2001. These and other enhancements opened up vast tracts of farmland to new housing developments.