DUI Speedy Trials: The Margin of Error as a Key to Acquittal
In this article
Key Takeaways
- Error margin 7.5%
- 0.60 mg/l threshold
- Widmark Formula
- Evidence nullity
A DUI conviction is not automatic: the breathalyser's margin of error is a real line of defence. For an in-service device, the maximum permissible error (Order ITC/155/2020) is 7.5% for rates above 0.40 mg/l; applied to a 0.64 mg/l reading, the proven rate can drop to 0.59 mg/l and fall below the 0.60 mg/l criminal threshold, turning the matter into a mere administrative infraction. This is combined with challenging the evidence where only a screening breathalyser was used, reconstructing the rate at the moment of driving with the Widmark curve, and seeking nullity where the driver was not informed of the right to a contrast blood test.
Need help with your case? Talk to a criminal defense lawyer at Alonso Sala.
In speedy DUI trials, conviction seems automatic. If the breathalyser ticket reads over 0.60 mg/l in expired air, an immediate plea agreement for the minimum penalty (fine and 8 months without a license) is often presented as the only realistic option to obtain the one-third reduction. At Alonso Sala, we examine each case before any plea decision: technical defence allows challenging the metrology of the devices and, where appropriate, breaking the presumption of the machine's veracity.
The Battle of the Margin of Error (PEM)
No device is perfect. Every breathalyser, no matter how advanced, has a Maximum Permissible Error (PEM). Order ITC/155/2020 regulates these margins, but the Prosecution systematically tends to ignore them or apply the "verification" margins (lower) instead of the "in-service" margins (higher).
The difference is vital. For an in-service breathalyser, the error margin is 7.5% for rates above 0.40 mg/l (or 0.030 mg/l for low rates). Practical example: If you tested 0.64 mg/l, the Prosecution will charge you with a crime. But if we apply the 7.5% error margin, the proven real rate could be 0.59 mg/l. By dropping below 0.60 mg/l, the fact ceases to be a crime and becomes an administrative infraction (fine and points, but no criminal record or trial). Fighting for those decimals is fighting for your freedom.
Evidential Breathalyser vs. Screening Device
The small device used at roadside checkpoints (screening or approach breathalyser) is not the same as the large "suitcase" found in the van or police station (evidential breathalyser). Only the ticket from the evidential one has full probative value. If a conviction has been based solely on the preliminary test, there is a route to challenge the judgment on grounds of violation of fundamental rights.
The Widmark Curve and the Ascending Rate
Alcohol does not affect everyone equally at all times. From the moment the last drink is consumed, the blood alcohol level rises for 60-90 minutes (ascending phase) and then falls. If an hour passed between driving and the test, it is possible that you were driving sober (initial absorption phase) and tested positive later (peak maximum). Through toxicological expert reports based on the Widmark formula, we can reconstruct the real rate at the moment of driving, which is the only relevant one for the crime.
Procedural Guarantees: Your Last Line of Defence
Many drivers are unaware of their right to request a contrast blood test at a nearby hospital. If the police did not adequately inform you of this possibility, or if they refused to transport you alleging lack of means, your right of defence has been violated. This defenselessness can lead to the radical nullity of the breath test, leaving the prosecution without evidence and forcing a free acquittal.
You can check which band the breathalyzer result falls into with our drunk-driving threshold checker.
Frequently asked questions
How does the breathalyser's margin of error work as a defence?
Every breathalyser has a maximum permissible error regulated by Order ITC/155/2020. For an in-service device, the margin is 7.5% for readings above 0.40 mg/l. If the result was 0.64 mg/l, applying that margin the actually proven reading can drop to 0.59 mg/l: falling below 0.60 mg/l, the fact ceases to be an offence and becomes an administrative infraction.
What is the criminal threshold for drink-driving?
A reading above 0.60 mg/l of alcohol in exhaled air. Below that figure the conduct is generally an administrative infraction (a fine and points), with no criminal record or trial.
What is the difference between a screening breathalyser and an evidential one?
The screening or approach breathalyser is the small device used at the roadside checkpoint; the evidential one is the equipment kept in the van or police station. Only the ticket from the evidential device has full probative value. If a conviction was based solely on the screening device's indicative reading, there is a route to challenge it.
What is the Widmark curve and why does it matter?
Blood alcohol rises for 60-90 minutes after the last drink (the ascending phase) and then falls. If time passed between driving and the test, toxicological expert reports based on the Widmark formula can reconstruct the real reading at the moment of driving, which is the only one relevant for the offence.
What happens if I was not informed of the right to a blood test?
The driver has the right to request a contrast blood test. If the police did not properly inform them of that possibility, or refused to transport them citing a lack of means, their right of defence is breached, which can lead to the nullity of the breath test and leave the prosecution without evidence.