Checkpoints DO tie up lots of personnel. In MD, the only people detained more than 30 secs are probable cause DUI's...folks who seem drunk. Usually between 1% and 3% of drivers get cited for DUI. And the checkpoints are operated between 2300-0300 only. So there are no mass impoundments, no license checks, etc. If the driver doesn't show some signs of intoxication, he/she is sent on without having to produce a license or anything else. BTW, most BAC's were .20 and up.
Yeah, different locatalities have different operating rules. If a roadblock is set up to check for DUI's, then logically, there shouldn't be vehicles being impounded if the driver shows no sign of drinking. But, this is Kaleefornia, and logic doesn't apply here.