In a nutshell, the answer is "no," but it is very difficult to defend these cases, at least for the most part.
Prior to the FSTs, many officers make some accommodations. They normally do not require people with prosthetic legs to perform the one-leg stand test and they sometimes give women the chance to take off high-heel shoes prior to the walk-and-turn test. If officers deny needed accommodations, the test results are skewed.
The Horizontal Gaze Nystagmus test is one of the three NHTSA-approved field sobriety tests. In almost all DWI cases, the FST results constitute probable cause for the arrest. If the defendant refuses to provide a breath or blood sample, the prosecutor must also use the FST results to establish intoxication beyond a reasonable doubt at trial.
At first blush, the one-leg stand test may seem like a rather mindless exercise. The ability to stand on one leg for fifteen or so seconds may seem unrelated to the ability to drive a car.
A hard-core fan blamed his team's ineptitude for a serious injury DWI crash outside the Meadowlands in New Jersey.
In most DWI defense matters, intoxication is the only issue. The state can use either direct or circumstantial evidence to prove intoxication. A chemical test that shows a BAC above the legal limit is proof of intoxication as a matter of law. Or, the prosecutor can rely on the circumstantial evidence from the field sobriety tests.
Normally, the walk-and-turn, along with the other approved field sobriety tests, establish probable cause for a DWI arrest. This evidentiary standard is between reasonable suspicion (an evidence-based hunch) and beyond a reasonable doubt. So, it's a rather low standard.
Most people do not associate "blow-and-go" Ignition Interlock Devices and the Eighth Amendment. But this provision prohibits "cruel and unusual punishments." Arguably, the IID falls squarely under that label.
Texas is an implied consent state. When people receive their drivers' licenses, they consent to chemical tests, specifically BAC breath tests. So, many people think that license suspension is automatic if they fail or refuse one of these tests.