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I never drink or do drugs, but I have tried the "heel-to-toe straight line" test and the "raise your foot for 30 seconds" test, and I find both of them to be difficult or impossible.

I have seen dozens, if not hundreds, of DUI tests on TV and the Internet, and while I understand that this is a skewed sample (they are probably more likely to broadcast the drunks), I don't believe I have ever seen anyone pass a DUI test.

Is there any science behind these tests? Are they just a blank check to arrest anyone? Even if you are later exonerated through a blood test, should you really have to go through the arrest process just because you're a klutz?

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NHTSA has guidelines implemented as a course and quiz which address this general concern. There are many indicators of intoxication, and failing too many of them is likely to result in an arrest. One Leg Stand, Heel-to Toe and Horizontal Gaze Nystagmus have repeatedly-confirmed legal potency (not necessarily universal, but valid in most states). Any line of evidence must be legally admissible in your state, if they are to be used in a trial. Some evidence such as smell of alcohol suffices as evidence for an arrest, but apart from blood alcohol measurement, the above three tests seem to be the only ones that generally suffice to result in a conviction.

In Washington v. Baity, 140 Wn.2d 1 the court concluded that

the underlying scientific basis for HGN testing – an intoxicated person will exhibit nystagmus – is "undisputed, even by those cases and authorities holding the test inadmissible without scientific proof in each case"

That does not mean that all HGN-related testimony is admissible. See Washington v. Quaale. Citing Baity, the court "placed limits on that testimony because the HGN test merely shows physical signs consistent with ingestion of intoxicants", and "an officer may not testify in a manner that casts an 'aura of scientific certainty to the testimony'", which the arresting officer did in Quaale – the officer overstated the scientifically-permissible results. To be admissible, the test must also be administered according to a specific protocol, that is, the officer must be specifically trained how to correctly administer the HGN test.

There is an analogous problem with an unrelated kind of evidence, "voice prints", which from a scientific standpoint are not reliable but from a legal perspective are not inadmissible (except in D.C). The legal standard for admissibility under US law is lower than you might hope. In Washington, the accepted standard is the Frye standard, and the evidence must be "generally accepted" by a meaningful segment of the associated scientific community. It is up to the (trial) court to determine if that standard is met, and appeals courts affirm those decisions unless they are "manifestly erroneous". Voice prints remain admissible because they are accepted by the scientific community of forensic voice print technologists (but not acoustic phoneticians). The scientific validity of fingerprints has never been established, but they are still admissible in court. Analogously, the field sobriety tests are generally thought to be reliable, in some relevant scientific community.

Here is a report commissioned by NHTSA, purporting to evaluate the accuracy of the three field sobriety tests. This is not peer-reviewed research. This article is a literature-review article, but it turns out that the studies summarized were similar commissioned studies and not peer reviewed scientific papers.

There is science behind the tests. Whether or not the science is solid is a different matter. The law does not impose stringent admissibility requirements on admissibility.

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I'm not a lawyer and this isn't legal advice.

Is there any science behind these tests?

If performed under correctly controlled circumstances by an unbiased, well-trained officer, they might be more likely than not to give the correct result. In reality, they are performed on the side of the road with flashing police lights, a nervous driver and a police officer that isn't going to waste their time administering them to someone they haven't already decided to arrest.

Are they just a blank check to arrest anyone?

An officer needs probable cause to arrest someone. An officer could have probable cause that a person is driving under the influence of a controlled substance without a field sobriety test, or could use the failed FST (according to the officer's administering of the test) as probable cause. It's possible that in some rare circumstance an officer has probable cause that someone is committing a DUI but decides that they were wrong after the person aced a FST, but that's not something that should be expected.

As far as I can tell, the relevant section in Oregon law for refusing a FST is ORS 183.136

If a person refuses or fails to submit to field sobriety tests as required by ORS 813.135 (Implied consent to field sobriety tests), evidence of the person’s refusal or failure to submit is admissible in any criminal or civil action or proceeding arising out of allegations that the person was driving while under the influence of intoxicants.

As you can see, the penalty for refusing a FST is just that the fact that you refused it can be presented as evidence against you, it doesn't carry a financial penalty or license suspension like refusing a chemical test. At least one attorney's office in Oregon (Romano Law) recommends that you almost never submit to field sobriety test (every other attorney's site I've found which makes a recommendation also recommends against taking a FST absent an attorney's advice for the particular situation):

Should I refuse a Field Sobriety Tests?

In most cases, yes. This is a complex area of DUI and search and seizure law, but in most cases, a police officer requesting Field Sobriety Tests has already made their mind up. You’re going to be arrested for DUI, and therefore they are only gathering more incriminating evidence of the crime of DUI with the FSTs. The FSTs are very difficult tests to perform without error, and anything you do incorrectly or poorly will be used against you to suggest you were drunk.

Ultimately, just about everything officers do during a traffic stop is intended to trick drivers into admitting to wrongdoing or provide evidence against themselves, including FSTs. Since you say you don't think you would pass them even if you are not under the influence of a controlled substance, it sounds like you may be better of not submitting to an FST (you are not required to explain why you are refusing the test either, and it is unlikely to be in your best interest to do so absent an attorney's advice), but if you are concerned you can contact a local attorney for a consultation.

Even if you are later exonerated through a blood test, should you really have to go through the arrest process just because you're a klutz?

The officer has likely already decided to arrest you before they ask to administer the FST, if it's any consolation.

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