Aug 12, 2019 by Foresight

Prescribed limits for drugs – the end to impairment testing?

Martin is a Forensic Physician (or Forensic Medical Examiner) and has been working with Foresight to provide expert witness opinions for the prosecution and defence. Here he discusses the history of drink and drug driving and issues of greater scrutiny around impairment testing.


The concept of being drunk in charge was introduced in the 1872 Licensing Act when the mode of transport applied to carriages, horse and steam engines. Fifty-three years later, the 1925 Criminal Justice Act penalised a person drunk in charge of any mechanically propelled vehicle on any highway or public place, and the 1930 Road Traffic Act introduced the notion of being properly drunk as to be incapable of having proper control of a vehicle.

The 1956 Road Traffic Act coined the term “unfit to drive” and the influence of drugs was added in the 1962 Road Traffic Act. Statutory levels for alcohol in breath, blood and urine were introduced in 1967 in response to a report from the British Medical Association written in 1960, when the breathalyser was introduced. Evidential breath testing as it is used today was incorporated in the 1988 Road Traffic Act.

Advances in technology

Advances in technology have influenced practice, for example the breathalyser resulted in the removal of the motorist’s choice to provide blood or urine, being referred to by Lord Bridge of Harwich in Warren as the ‘old code’. The ‘new code’, set by precedent in Horricks v. Binn, established the principle that it was unlawful for the officer to require a sample of blood or urine when there were no medical reasons why a sample of breath could not be provided and a reliable breath analysis machine was available.

This progression toward reliance on technology has continued with the removal of the “statutory option” of drivers whose breath alcohol level was above the prescribed limit of 35microgrammes of alcohol per 100mls of blood, but below 50microgrammes of alcohol per 100mls of blood. This move was predicted to save the Police around £12.5m per year and accrue approximately £1.3m in court fine revenue.

Impairment testing for alcohol

Field Impairment Tests (FIT) have been validated in the US where the ability to walk a straight line, stand on one leg for 30 seconds, along with the officer’s observations of the size of the pupils and any movements of the eyes …eyes when looking to the left or the right, are admitted as evidence.

In the US, impairment tests are often referred to as sobriety tests and, when standardised, have been shown to predict blood alcohol levels. There are stricter guidelines when applying the tests and recording “clues”. For example, there is an age threshold whereby it is accepted that a person aged over 65 may have difficulty with the Walk and Turn test. When standardised – if two or more clues are present – including a loss of balance when the instructions are given, walking too soon, stopping when supposed to be walking, missing the heel to toes position by more than ½ inch, raising the arm by more than six inches, stepping off the line, turning improperly, or taking the wrong number of steps, then this test points towards a particular blood alcohol level.

In the UK, Field Impairment Tests include distraction elements where the individual’s attention is divided between a mental and physical task. For example, in the Romberg Test, the individual is asked to stand with hands down by the side, eyes closed and head titled backwards, bringing the head forward when the individual thinks that 30 seconds has passed. The distraction in the Walk and Turn test is the requirement to call the out the number of steps taken along the line and similarly in the One-leg Stand test the individual has to count out time by saying out loud “one thousand and one, one thousand and two and so on”. The final task is for the individual to place the tip of the index finger on the tip of the nose whilst the examiner records the position of the part of the nose touched on a body map of the face.

The influence of drugs

Although the influence of drugs on driving was recognised in the 1962 Road Traffic Act, development of procedures to deal with driving under the influence of drugs were established at a much slower rate. This is in part due to the recent introduction of screening devices to test the saliva – at the roadside – of those suspected of driving under the influence of drugs.

Drug testing

Given this, it might appear that blood levels of drugs provide the best evidence for the prosecutor (urine levels of drugs cannot be used for evidential purposes because (some) of the way that drugs are processed in the body). So blood test for drugs that are clearly abused, for example cocaine and cannabis – for which there are no medical reasons for their use – are the only option. Since the effects of cocaine and cannabis may be short lived, lasting minutes rather than hours, it is possible that persons may not appear to be impaired but still have substances in the blood that exceed the limits specified in legislation.

Expert witness assessment

Perhaps it is the notion that tests have been validated that explains the emphasis that is placed upon them. The impairment tests should only be a part of the assessment made by the Forensic Medical Examiner or other healthcare professional in response to a request to “ascertain whether in your opinion there is a condition that might be due to drink or drugs”.

The general medical and forensic training of a forensic medical examiner should identify conditions such as multiple sclerosis that might account for difficulties standing on one leg, or walking an imaginary line. The presence of widespread infection, from a bladder condition or a septic injection site, may produce a state of confusion that can be mistaken for intoxication.

Until the result of saliva testing reaches evidential reliability, impairment testing will remain as the main method of assessing whether an individual has a condition that might be due to drink or drugs. As lawyers become more familiar with drug testing it is likely that those conducting impairment tests will face greater scrutiny of their findings.

Solicitors faced with alcohol or drug related cases should seek expert opinion to validate the factual statements made by professional witnesses such as forensic medical examiners and nurse who were present at the time of observation.


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