With cellphones, twitter alerts and text messages roadblocks are virtually impossible to surprise the hardcore drinkers. There are number of underground twitter feeds that, for the most part, accurately report most of the FFX county roadblocks with in minutes of being setup. The problem is the roadblocks must remain in the same spot for a couple of hours.
There's also Trapster and mobile apps that offer DWI roadblocks alerts. Not to mention that everybody at nearby bars are easily alerted to road blocks by bartenders and other patrons. I've heard people talking about roadblocks on the complete opposite end of the county.
This a poor example
http://www.duiblock.com/dui_checkpoint_locations/virginia/ If you google good you can find the good alert sites and twitter feeds.