On a real level, I'm not sure I'd want a Cop to take my blood, where are the checks and balances? Say a typical weekend nabs twenty drunk drivers, average fine is say $2000, in a year that could add up to a couple of million bucks. If the people doing the arresting also collect and control the evidence, where is the protection for the citizen? The potential and motivation for Police fraud is there. Take say twenty samples from some wino, slap any old name on there and submit it as evidence, presto chango and extra 2 million a year.
I was pulled over in a drunk check, guy had me blow, then said I'd have to give blood. I'm sitting in the hospital waiting room with a dozen drunks thinking it sure would be easy to mix up a blood sample here ( I hadn't had a drink in months). I demanded two blood samples be taken and payed for the second myself. I was really afraid they'd screw it up and mix up the samples.
The roadside check was a circus, people parked all over, officers and citizens wandering all over. The hospital was about the same, a three ring circus.
If they tested every sample for not only blood alcohol, but for also say PSA, they may also save lives with early cancer detection. Just a thought.