In article >, says...
>

>Not here (DC and VA) they don't. They block off a road and stop
>everybody. Now they only give "extra attention" to those actually
>appearing inebriated, but EVERYONE has to stop, there's no flagging down
>of every 3rd car or anything, it is a hard checkpoint.

They have a choice. Since in most places stopping every car would really
screw up traffic royally, they stop every X cars. I'm going to guess that
the road where they stop everyone is not one that has alot of traffic.
--------------
Alex