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I have been watching a number of "cops arrest drunk driver" videos lately. In every case, they don't even mention that "breathalizer" or whatever they called it, but instead go into these elaborate tests that take forever, where they make the suspect walk in a line and follow their finger with their eyes and stuff like that.

Is there any reason that they do this now? There probably is. Was the "breathalizer" found to be inaccurate or something? Do they really not have a simpler and quicker way to check if somebody is drunk/impaired in the year 2021?

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    For your research, these are called field sobriety tests. – Nate Eldredge Jun 3 at 23:04
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    I think the answer may lie in the fact that videos of some people's inability to put a finger on their nose or walk in straight line is much more entertaining than the anticipation of a red or green LED being lit up. – Rock Ape Jun 4 at 7:41
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Breathalyzers are an objective test, if they come back negative they are exculpatory evidence for the defense. On the other hand, field sobriety tests are entirely judged by the officer at the scene, and until the prevalence of body/dash cameras, the officer's word was the only relevant testimony to whether or not the person passed or failed (and for the Horizontal Gaze Nystagmus test, i.e. the follow the pen test, it's still based pretty much entirely on the judgement of the officer). So, FSBs are very helpful for the prosecution because they are effectively always inculpatory, since there's rarely a way to impeach the officer's testimony that the person failed.

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Driving "under the influence" doesn't always mean alcohol, field sobriety tests check for impairment, breathalyzer measures alcohol levels in the body. You can be impaired without being drunk.

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    High on illegal drugs, for example. Or drowsy because of perfectly legal medication. Or just recently I had an eye examination where they put drops into my eyes that would have made it quite hard to drive. No idea how I would have done in a "field sobriety test". – gnasher729 Jun 4 at 9:03
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    In E&W, it is an offence to drive on the road if one's ability is impaired by perfectly legal medication or eye drops - s.4 and s 96 of the RTA1988 respectively. – Rock Ape Jun 4 at 9:39
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    Well, me wife had to drive me :-) (They warn you when you get your appointment). Obviously a breathalizer won't detect these things. – gnasher729 Jun 4 at 13:51
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Field breathalyzers are potentially unreliable, and in fact are inadmissible in some states (e.g. Kentucky, KRS 189A.104), where the chemical test must be performed at the police station. There are training and calibration issues involved with the field machines, which are overcome with the in-station trained staff and well-maintained machines. Part of this procedure is waiting 15 minutes to be sure there is no alcohol in the mouth. Additionally, the more sophisticated big machines can specifically measure ethanol, not just methyl group structure which may reflect the fact that you were just painting. These factors weigh against using field breathalyzers: they are more challengeable.

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    Im not convinced by the argument against field breathalizers - they are the standard in many countries and are very successful. In the UK, they give an officer enough basis to arrest a person and detain them for an evidential breath test at the police station (basically, in the UK, a constantly calibrated and tested non-portable breathalizer that takes multiple samples) - there can be no argument about an officers judgement, and if not backed up by an evidential test there is typically no prosecution, so theres no argument about the field units accuracy. – Moo Jun 4 at 1:55
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    Nothing prevents the use of field breathalyzers as a first step in establishing probable cause for further action. In Spain the solution is that the officer must A) perform the test a second time at least ten minutes later to confirm the results and B) if the confirmation test still shows too much alcohol, the accused is offered the possibility of requesting a blood test (at the accused's expense if it still shows that the accused has indeed been DUI). – SJuan76 Jun 4 at 7:03

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