That kind of thing has been suggested before, the problem is creating a viable sensor that can tell if someone is actually above the legal limit reliably.
There is a reason why those breath testers are only used as the first step, and indicator that needs to be confirmed with a blood test. They just are not that reliable or accurate and it would be a problem if your car wouldn't start because you took some medication that caused a misdiagnosis.
Also it would be problematic from a practical point of view. The breath testers have disposable parts for you to put your lips around, they are used once and discarded. Creates a lot of waste plastic. Cleaning might be an option but especially these days with COVID...
And after all that it wouldn't detect things like drug use or tiredness.
As usual this
totally misses the point. It's not blood alcohol
per se that is the concern, it's the
effect it has on the ability to drive. My wife has a phenomenally low tolerance for alcohol, and after literally ½ glass of wine (making her way below the legal limit) she is
truly incapable of driving. Conversely, I can tolerate far more (really). This means that, according to the law, if we go out to a bar it should be my incapacitated wife who drives, not me. Similarly, diabetics can sometimes lose control of blood sugar, which produces very similar effects to alcohol intoxication, yet legally they can still drive.
If you want to be scientific about driving "while under the influence" you should look at the
direct effect on driving competence, not an averaged "one size fits all" check of blood chemistry. As has been noted many times in the past, the
correct way to test is some form of reaction testing. To an extent, the US informal sobriety tests (walking a line, touching fingers together etc), inaccurate and humorous though they are, and a far
better indication of driving capacity than blood alcohol level.
This is a classic case of indirect regulation getting out of control. "We want to regulate B (driver incapacitation). A (alcohol) causes B. It's easier to regulate A than B, therefore we regulate A." This fails when (a) A is not the
only cause of B, or (b) when A only
sometimes causes B.