The locations are random but that doesn't violate any constitutionality requirements. Randomness only matters to the extent that they are selecting drivers randomly. So long as every car that passes through the checkpoint is stopped - there is no random search.
The metal detector equivalent is the same. Metal detector locations are random. They are set up whenever and where ever someone feels there's a need. They are not random in that everyone must go through them. Random locations are fine, random people are not.
Detention is also in favor of checkpoints. The Court has frequently weighed the extent of the intrusion and upheld cases where the intrusion is not substantial. Motorists are not obliged to pass through checkpoints and can turn their vehicles around to take alternative routes to their destination. More importantly, the length and depth of the intrusion is minimal compared to the government interest in stopping drunk driving.
I have to disagree with the bolded part. The balancing test has only been applied in cases where the stop is substantially less intrusive than a typical arrest. Otherwise, the general rule of probable cause is required to stop and seize a person. I'd posit that such a stop and seizure is not substantially less intrusive than a typical arrest, but I'd assume you will differ.
However, even conceding that such a stop and seizure is substantially less intrusive, and thus allowing us to use the more lenient 'balancing test', the reasonableness of the seizure must be judged by balancing the gravity of the public concerns served by the seizure, the degree to which the seizure advances the public interest, and the severity of the interference with individual liberty.
I think the government's arguments fall short in every prong.
Also, to your point about the difference between border searches/stops and dui stops.....
Requiring borders stops to be based on reasonable suspicion would be impractical because the flow of traffic tends to be too heavy to allow the particularized study of a given car that would enable it to be identified as a possible carrier of illegal aliens. It's often easy, or at least easier, to tell when someone is driving drunk as opposed to driving while being an illegal alien. This is entirely different when it comes to gathering individualized suspicion to stop suspected drunk drivers. Normal (constitutional) law enforcement measures are more than enough to detect suspected drunken drivers whereas the same cannot be said for illegal immigrants traveling in vehicles.
In other words, I would need to see evidence that there is a similar difficulty in detecting individuals who are driving under the influence of alcohol, because it is not intuitively obvious that such a difficulty exists (as it is in the case of border stops).
Without proof/evidence that law enforcement cannot develop individualized suspicion that a person is driving while impaired by alcohol, the constitutional balance must be struck in favor of protecting the public against even, if it must be conceded, "minimally intrusive" seizures.
