DUI & Traffic Accident Center · Procedure Guide

The measured reading and your BAC while driving
may not be the same

BAC rises for a while after drinking and then falls gradually. If the time you drove and the time you were tested differ, the reading sometimes cannot be taken directly as your BAC while driving. In a case on the 0.03% or 0.08% borderline, this must be examined.

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Key Points

  • BAC generally peaks 30 to 90 minutes after drinking and is then known to fall by 0.008% to 0.03% per hour, about 0.015% on average.
  • If you stopped driving during the rising phase, your BAC while driving may have been lower than the reading.
  • However, the rising phase alone does not make proof impossible; the time gap, the difference in readings, the amount of alcohol, the manner of driving, and how any accident occurred are considered together.
  • To use the Widmark formula, the underlying facts such as the amount of alcohol, body weight and sex must be strictly proven, and values favorable to the defendant must be applied.

1. The problem with readings taken in the rising phase

The Supreme Court holds that even if a reading taken well after driving ended slightly exceeds the legal threshold, it cannot be concluded that BAC exceeded the threshold at the time of driving if driving may have occurred during the rising phase. However, the mere existence of circumstances suggesting a rising phase does not make proof impossible. The position of case law is that the interval between driving and testing, the difference between the reading and the threshold, the duration and amount of drinking, behavior at the stop and the test, and how any accident occurred must all be considered together (Supreme Court Decision 2013Do6285, decided October 24, 2013).

That is why it is important to pin down the following times to the minute.

Times to establish and supporting evidence
WhatHow to prove it
Start of drinking and the last drinkCard payments and order records, statements of the people present
When driving started and endedDashcam, parking lot entry and exit records, navigation records
The stop and the testReport on the drunk driver's condition, test result printout
If blood was drawnTime of the blood draw, the forensic analysis report

2. What is the Widmark formula?

When you could not be tested because you were taken to a hospital after an accident, or the test was taken long after driving, investigative agencies use the Widmark formula to estimate your BAC at the time of driving backward. The amount and strength of the alcohol, body weight, the sex-specific Widmark factor, and the elapsed time are entered into the formula.

Case law holds that to use this formula, the basic facts such as the amount of alcohol, the time of drinking and body weight must first be rigorously proven, and that for the factor and the hourly elimination rate the value most favorable to the defendant must be chosen. A loosely stated amount of alcohol often becomes the starting point of the calculation, so during questioning you should state only what you remember with certainty.

3. Breath tests versus blood tests

  • Breath test · The basic method used at the checkpoint. Residual alcohol in the mouth can produce a high reading, so the test should be taken after rinsing with water.
  • Blood test · If you cannot accept the breath test result, blood can be drawn with your consent and tested again (Article 44(3) of the Road Traffic Act). A blood test takes more time, so the reading may be lower because you have entered the falling phase, or higher because you are still in the rising phase.
  • When the two results differ, the blood test result is usually considered more accurate. So before asking for a blood test, you should consider when you last drank.

Frequently Asked Questions

I drove right after drinking and was tested 30 minutes later.
You may have been tested in the rising phase. If the reading is near the 0.03% or 0.08% borderline, you can argue that your BAC may have been below the threshold while you were driving.
I could not be tested because I was taken to a hospital after an accident.
Your BAC will be estimated using the results of blood drawn at the hospital or the Widmark formula. The issue is whether the amount of alcohol and the times used in the calculation are accurate.

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Written on the basis of Korean statutes in force (National Law Information Center, Ministry of Government Legislation) and court precedents as of October 20, 2026. Results may differ with amendments to the law or the facts of each case. This page is general information, not legal advice.

Written and reviewed by Attorney Dongwoo Shin — Ph.D. in Law; certified criminal law specialist and certified divorce law specialist registered with the Korean Bar Association. He handles each case personally from consultation to judgment. About the attorney →
Last updated October 20, 2026

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