In Great Britain a legal blood alcohol limit of 80 mg per 100 mL has operated since 1967. Since the introduction of evidential breath testing with a legal limit of 35 µg per 100 mL, the law has enabled those with breath alcohol levels below 40 µg to escape prosecution and those with levels of 40 to 50 µg to opt for a blood or urine sample. The system means that marginal subjects are not prosecuted due to a variety of factors including time delays and analytical tolerances.
This paper draws on analytical results and breath/blood interconversions to show how the operation of the law works to the benefit of the drink-driver due to time delays in sampling. It demonstrates that in countries in which the blood option applies it means that the real drink-drive limit is about 20 mg above the legal limit.
The taking of samples of body fluids for analysis in drink-drive cases has been permitted since 1962 in Great Britain (HMSO, 1962), but it was only following the Road Safety Act 1967 (HMSO, 1967) that practical procedures for the taking of evidential blood and urine samples were defined along with non-evidential roadside screening tests. The introduction of evidential breath testing at the police station did not occur until after the Transport Act 1981 (HMSO, 1981). Although powers have existed to change legal limits for alcohol in blood, urine and breath, the accepted values in Great Britain are still those introduced in 1967: 80 mg in 100 mL of blood and 107 mg in 100 mL of urine, and the 1981 value of 35 µg in 100 mL of breath.
However, the procedures employed for taking the samples and the subsequent analyses have led to so many allowances, tolerances and delays that the effective legal limit in Great Britain is of the order of 110 mg in 100 mL of blood.
For a person to be prosecuted for being over the legal limit the following steps normally occur:
1. Motorist stopped and roadside screening test applied (positive result cannot be used for prosecution).
2. Motorist taken to the police station.
3. Police station procedure commenced.
4. Evidential breath test procedure carried out.
5(a) No further tests or samples permitted if subject provides two samples with both values being in excess of 50 µg (equivalent to 115 mg for blood).
5(b) Motorist released if lower of the two recorded breath alcohol values is below 40 µg. Prosecution does not occur if the recorded value is in the range 36 to 39 µg although the legal limit is 35 µg.
5(c) If the lower result is between 40 to 50 µg the motorist has the option of requesting that a blood or urine sample be taken to take the place of the breath alcohol value.
6. A Forensic Medical Examiner (FME) then has to be brought to the police station for the purpose of taking any blood sample.
7. When the blood sample is analysed an analytical allowance (for possible statistical errors in the analysis) of 6 mg (or 6% ) is made. As a result, no prosecution occurs if the analysis produces results between 81 and 86 mg in 100 mL of blood.
It is, therefore, very clear that the law and the procedures employed in Great Britain serve to introduce delays and allowances that can only work to the advantage of the motorist and lead to effective alcohol levels well in excess of those intended by the supposed legal limits.
The above list shows that there are two main features which lead to delays in taking samples. The first one is the time taken to get the motorist to the police station, and the second one is in getting the FME to come to take the blood sample. Following this, the subtraction of 6mg from the analytical result bends the whole system further to the motorist's benefit.
In other words, a person with an evidential breath alcohol value of 42 µg can be almost certain to avoid prosecution if a blood sample is taken after a normal delay of 42 minutes. Even a person at the 50 µg level will still avoid prosecution if the delay in taking the blood sample exceeds 1 hour 56 minutes.
The effective legal limit in Great Britain for drinking and driving is closer to 110 mg than the official limit of 80 mg. As a result of the allowances and delays which occur at present motorists who are well above the legal limit when they are driving are avoiding prosecution. The problem can only be overcome by legislative changes of the following types:
Road Traffic Act, 1962, HMSO, London, United Kingdom, (1962).
Road Safety Act, 1997, HMSO, London, United Kingdom, (1967).
Transport Act, 1981, HMSO, London, United Kingdom, (1981).
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