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Factors Affecting Non-Prosecution of Over the Limit Motorists in Great Britain

Dr. Ronald C. Denney

School of Biological & Chemical Sciences, University of Greenwich, Wellington Street, London SE18, UK


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 g per 100 mL, the law has enabled those with breath alcohol levels below 40 g to escape prosecution and those with levels of 40 to 50 g 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 g 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 g (equivalent to 115 mg for blood).

5(b) Motorist released if lower of the two recorded breath alcohol values is below 40 g. Prosecution does not occur if the recorded value is in the range 36 to 39 g although the legal limit is 35 g.

5(c) If the lower result is between 40 to 50 g 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.

  1. Delay between arrest and test: This is a highly variable factor, and a study of cases with which I have been involved has shown that the time taken can range between 25 and 77 minutes, with an average of about 40 minutes before the evidential breath testing is carried out.
  2. For the motorist with an evidential breath alcohol value in the 40 to 50 g region there is a clear advantage to be gained by accepting the legal option and asking for a blood sample to be taken. There will almost certainly be some delay in getting the FME to the police station to take the sample. During that time further alcohol elimination will occur.
  3. In an evaluation of cases involving both breath and blood alcohol values, I found that in 68 drink-drive cases in which subjects provided evidential breath alcohol values between 40 and 50 g and then supplied blood samples, for the period 1st January 1991 through to 31st December 1994, the average delay between the breath tests and the blood sampling was 42 minutes. The delays varied from 11 minutes to 120 minutes. Rapid blood sampling was usually only likely if the FME was already on the premises for other purposes, and this was very uncommon occurring only in about 3 per cent of the cases.
  4. The other important point in this connection is that in the vast majority of cases, back calculations from the blood analysis value, after allowing for the 6 mg analytical tolerance and the time difference (using a metabolic rate of 15 mg every hour and a 1:2,300 conversion factor) gave breath alcohol figures corresponding almost exactly with those obtained for the evidential breath samples. This meant that virtually all the subjects were in the elimination phase for the period they were at the police station during which samples were being taken.
  5. The 6 mg analytical allowance on the blood alcohol value is an anachronism which has its origin in the wet analytical chemical procedures originally used for blood analyses after 1967. It was a statistical allowance made for the standard deviation of the procedures at that time. As all police samples are now analysed by gas chromatography, with a much smaller standard deviation, there is no doubt that this allowance should be drastically reduced.
  6. The Cumulative Effect If a motorist has reached the elimination phase in the alcohol cycle by the time the roadside test is carried out then the cumulative effect of the delays and allowances must work substantially to his or her advantage and against the general principle of prosecuting people with breath alcohol levels in excess of 35 g. The average delay in getting to the police station corresponds to about 10 mg, the average delay in obtaining the FME corresponds to another 11 mg and then a further 6 mg is subtracted from the analytical result. So on the average 27 mg will have been lost from the blood alcohol level of a motorist who is in the elimination phase and records an evidential breath alcohol value marginally above the legal limit.
  7. Motorists avoid prosecution if the blood sample eventually produces a certificated value of 'not less than 80 mg in 100 mL of blood'. This corresponds to a true analytical value of 86 mg. But it also corresponds to a blood alcohol value of 97 mg at the time of the evidential breath testing or 107 mg at the time of the roadside test (back calculated and based upon average delays).

In other words, a person with an evidential breath alcohol value of 42 g 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 g 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:

  1. By reducing the legal limit to 50 mg in 100 mL of blood. In the absence of any other changes in the law this would still mean that the effective limit would be approximately 80 mg.
  2. By reducing the analytical tolerance to about 2 mg in place of the present 6 mg, in order to be more commensurate with modern analytical procedures.
  3. By allowing prosecution to take place on the earlier breath sample values even if the later blood sample figure is 80 mg or below, whilst placing the onus on the defence to show that the breath and blood alcohol values are incompatible.
  4. By the use of digital recording roadside evidential testing equipment.


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).