Why do US police do field sobriety tests?

I don’t know if there is a GQ answer to this one.

If TV is to be believed (I know), when testing for a DUI, most police officers in the US appear to carry out what I believe is called a field sobriety test. (Walk in a line, touch your nose, etc).

Assuming that is the case (I stand to be corrected). Why go through that whole rigmarole, when a handheld breathaliser can tell with reasonable accuracy within 5 second what someone’s BAC is? The driver doesn’t even need to get out of the car.

To add a question, If someone is brought in after a field test, is there any subsequent testing done? Or is the officers testimony considered enough?

I can speak about it in Canada, which is similar to the US in this area.

Not every single police car has a roadside screening device. If an officer sees a car driving with signs of impairment, the officer had to investigate, even if the officer doesn’t have a screening device. Sobriety tests are one way for the officer to assess whether the driver is impaired.

If the officer concludes that the sobriety tests provide reasonable grounds to believe the driver is impaired by alcohol, the officer can them give a demand for a breath sample and take the driver to the depot for a test. The results of that test will be used to determine if the driver is driving over .08. The officer’s personal observations of the driving conduct will be used as evidence in the charge of impaired driving, which is a separate charge. The officer’s observation of the sobriety test is not admissible on the impaired charge, since it was compelled without giving the accused the right to counsel.

Missed the edit window.

Meant to clarify that my comment that Canada and US are similar applies to why the sobriety tests are used.

There are differences between Canada and the US in how the evidence of sobriety tests can then be used in subsequent charges. Maybe one of our US LEO or lawyer Dopers will be along to speak to that.

IANAL, but is seems the sobriety test is also a decent way for testing someone’s awareness and level of impairment regardless of their BAC. If you blow less than the allowed, you could still be driving impaired for a lot of other reasons (e.g. you’ve been up for 3 days straight).

I agree though, it doesn’t hurt to have a breathaliser on-hand and use it if you have it.

I have seen a few DUI cases while I was in traffic court for something, and generally the testimony that the subject failed the field sobriety test is followed by testimony that the subject was taken to a brethalyzer machine for more precise testing.

Apparently the brethalyzer test is a bit complicated, and one of the key elements is that the officer needs to observe the subject during the entire test to verify that the subject didn’t vomit, as regurgitating even a small amount will compromise the test.

I know blood tests are sometimes used, but I don’t know why. As in, is a that some jurisdictions use blood tests and others a breathlizer, or whether it is based on circumstances, or whether it is at the officer’s discretion: I just don’t know.

In the United States a police officer generally needs to have probable cause to believe you have committed a crime in order to arrest you. The primary purpose of the FST is to establish probable cause. Criminal cases have a certain process, and if the State hasn’t done things correct at any point in that process the entire case can unravel. So if the police officer pulled you over without probable cause, then it’s highly likely anything found after that will be inadmissible and the case would be dismissed.

If they pulled you over for a broken tail light but after speaking with you believed you were drunk, they would need probable cause to arrest you. Police are good at covering their asses, so every arrest report on DUI will also typically contain phrases like “suspect smelled of alcohol, was slurring their speech, had glassy eyes” because even without a FST or portable breathalyzer test just the observation that someone appears intoxicated can also establish probable cause. But you want as much PC as possible so if the suspect is willing to do an FST and a breath test you’d certainly want them to, as failing those in addition with the report about how the person appeared intoxicated would get you easily over the probable cause bar.

In the United States though, FST and PBT results are only legally useful for establishing that the officer had probable cause to make an arrest if it wasn’t at a checkpoint they’d also need PC for the stop itself–like the vehicle weaving in and out of traffic, running a red light, having expired tags etc.

Once you get to the police station they have a much larger breathalyzer machine that can be used for evidentiary purposes. These machines are supposed to be rigorously maintained, and calibration records kept etc so there is more of a technical basis for trusting their results although a blood draw is the only way to accurately know BAC, because measuring breath concentration can actually misrepresent blood concentration in some individuals–I personally do not believe breath testing should be admissible at all and the only use of it should be the PBTs that can establish PC. For the actual evidence the police should be required to draw blood for chemical analysis.

I think of an FST as a screening test. If you pass, you drive on; if you fail, they bring you in for a more definitive and admissible test.

It has sort of been implied, but I’ll still say it: not all impairment is due to alcohol. Failing a FST while having a BAC of 0 doesn’t mean you’re OK to drive, it means you might be impaired by something else - exhaustion, illness, various pharmaceuticals both legal and illegal…

It’s illegal to drive impaired, regardless of what is making you impaired.

(slight hijack)
The BAC for intoxication for a Commercial Motor Vehicle Driver (Truck/Bus/etc) is 0.04
(slight hijack)

Wow. The general BAC in Australia is 0.05, and commercial drivers must maintain a BAC of 0.00

In traffic school, I learned that it is perfectly legal in Illinois to refuse the field sobriety test. You can not refuse to give a sample of your breath or blood, but you can refuse the field sobriety tests. The officer has already decided to arrest you anyway, you are only providing evidence against yourself. You can assume your field sobriety tests are being recorded and the district attorney can choose to introduce them as evidence if she or he chooses to do so.

I’d be so nervous, I’d have a hard time reciting the alphabet stone cold sober on the side of a highway with police lights flashing behind me.

Police need several things:

  1. Observable driving violations so they can stop the vehicle & issue a ticket (weaving in lanes, variations in speed , broken equipment, etc.).
  2. Probable cause to believe that the driver is impaired (indications of drinking, fatigue, drug use, etc.)
  3. Recordable evidence that can be used in court (field sobriety tests, done in front of a dashcam video).
  4. Additional evidence for court (portable breathalizer test, done by a different officer, who is trained in DUI cases).
  5. Further evidence for use in court (recorded test on a breathalizer machine at the station by a trained technician, or a blood test done by medical & laboratory personnel).

So the whole procedure is a series of steps to concentrate on actual impaired drivers, with each step providing more specific evidence, with more witnesses and increasingly accurate & recorded test results.

And it’s quite effective – very few people arrested for DUI ‘beat’ the charge. You can see this from the actions of lawyers specializing in DUI cases – they put their efforts mostly in trying to exclude evidence from court, or in trying to bargain the case down to a lesser charge.

I do not believe that is true. When I received my CDL, the literature said that any measurable amount of alcohol was over the limit. This includes when driving our personal, non-commercial vehicles. We also agreed to provide a blood sample if involved in a collision.

That may be from state to state I guess. Here it is .04%.

Well, you can refuse the breath and blood tests (at the station), but then you automatically get your license suspended. I imagine this is true in all US states.

I suppose there could be a situation where someone might opt to lose his license rather than give proof of his intoxication (with regard to the test results).

ETA: And refusal can be used as evidence of guilt in court? IANAL.


This is not true. The FST is a tool for determining impairment. I have given numerous and not made an arrest.

Can you provide a cite on that?

I’ll add that I have received numerous FSTs, as well as roadside breathalyzer tests, and never been arrested.

On one occasion the officer said something like ‘well, you borderline failed the eye test, but maybe you just have astigmatism (which I do), so let’s take a breathalyzer and clear it up.’ Something to that effect anyway, I don’t remember the exact words.

Federal Motor Carrier Safety Administration
Regulation 382.201 Alcohol concentration
“No driver shall report for duty or remain on duty requiring the performance of safety sensitive functions while having an alcohol concentration of 0.04 or greater.”

Some states have more stringent rules for CMV drivers, but this is the Federal Guideline.

(Hijack over)

Nobody passes. Don’t do the SFST. Even if you think you do great, they’re almost certainly filming it and they will get the officer to “point out” all the mistakes you made on the video. Even if those mistakes don’t actually exist, the jury will believe the cop in the uniform. The prosecutor will talk on and on about drunk people killing children and you’ll almost certainly be convicted. Better to not give them the evidence even if you’re perfectly sober.

In Houston, the cops go after sober people because they’re more likely to go to trial, and the cops get paid double overtime to be in court (even if it’s during their working hours). Every cop on the HPD DWI task force makes over $100k per year in double overtime “DWI testifying” pay. Guilty people who are blatantly drunk tend to plead guilty and there’s no need for a trial, thus no need for the officer’s testimony. Thus no extra pay.

I think it’s like this in many other cities, too.


I agree with kaltkalt. If you have had ANY alcohol, it’s best to refuse the FST. If the cop has evidence you’re drunk, he’ll arrest you. If he doesn’t, why provide him such evidence doing circus tricks that are designed for failure. It’s basically a lose-lose proposition. You are not required to submit to a FST in most states, and can refuse one. If the cop decides to move on, refusing a real breathalizer test or a bloot test is the same as failing and you are presumed guilty and will lose your license.

For clarification, the FST is not evidence that someone is drunk, and is not proof of guilt. Proof is provided by the expensive breathalizer back at the station or by a blood test.