In Los Angeles, there is more than one way to prove a DUI. Suppose they believe that you have alcohol in your system and can't safely operate a motor vehicle. In that case, they can charge you under California Vehicle Code 23152A, which doesn't have anything to do with what percentage of alcohol is in your blood.
The other they can arrest you is if you test at a 0.08 or greater. There is a presumption in the law that you cannot safely operate a motor vehicle at that point.
Once you do the field sobriety tests and police conclude that you are under the influence of alcohol and cannot safely operate a motor vehicle, it will justify them placing you under arrest and bringing you into the station to do a blood or breath test to see if your blood alcohol level is a 0.08 or greater.
Can Anything I Say In The Police Car Be Used Against?
A lot of people get excited about their Miranda rights. They don't realize that the police don't need to read anyone their Miranda rights a lot of times. They don't need to do anything other than a book you for a DUI and then let the prosecutors deal with it from there.
The Miranda rights come into play after they've decided to arrest you. You're in custody, and now they're going to start to ask you questions to try to get incriminating answers. If they don't read you your Miranda rights under those specific circumstances, then any statements you make that might incriminate you will not be able to be used against you.
I see people's Miranda rights being violated in DUI stops, but most of the time, the police have the evidence they need, regardless of your statements. Even if your statements get thrown out, if you got pulled over and your blood alcohol was double the legal limit, they have enough evidence.
When police pull you over, you're not necessarily in custody; you're being detained pending an investigation. Once they slap the cuffs on you and you're in control, any direct questions require your Miranda rights to be read. If they aren't and you answer those questions, those answers cannot be used against you in your DUI prosecution.