First of all, unless the substance being measured is some type of drug, blood alcohol content is in fact the only thing that is used to determine a person’s sobriety. After all, you can’t be drunk without having alcohol in your blood. The question is how you measure this blood alcohol content.

A blood test measures alcohol concentration directly. A breathalyzer serves as a surrogate measure for blood alcohol content. A still more indirect measure, as you suggest, would be to look at failed field sobriety tests, erratic driving, and other signs of impairment. At the same time, the purpose of all these measures — both direct and indirect — is to determine whether or not the person is intoxicated. The intoxication can only occur if there is alcohol in the person’s blood. Therefore, the purpose of all these measures is to try to determine blood alcohol content of the person being investigated.

I also agree that many jurisdictions allow for DUI conviction for driving with blood alcohol concentrations below the statutory limit. That’s why, as my mother says, any number of drinks CAN be too many.

The jurisdictions in which this is possible would include Washington, D.C. As I have spelled out elsewhere on this site, D.C. differentiates between driving while intoxicated (DWI), on the one hand, and driving under the influence (DUI) and operating while impaired (OWI), on the other. DWI is a per offense in which, in order to secure a conviction, the prosecution need only prove that the defendant was in operating or in physical control of the vehicle while his/her blood alcohol content was above the statutory limit. This is in contrast to DUI and OWI. With respect to those two offenses, the prosecution must prove impaired driving through intoxication in addition to operation/physical control. And it can try to do this, as you suggest, through evidence of failed field sobriety tests and erratic driving.