Are the Preliminary Impairment Tests used by UK police fit for purpose?

UK police officers make decisions about whether a person’s ability to drive properly is impaired. Those decisions are accepted by courts as evidence of driving-ability impairment. Those decisions are based on police officers’ subjective assessments of a set of tests that were not designed or validated to detect driving-ability impairment and with no established baseline of performance. This editorial discusses the origins of the Preliminary Impairment Tests used in the UK to establish if a motorist’s ability to drive is impaired through alcohol or drug use. The focus of this editorial is how these impairment tests have never been validated against a control subject group, nor have they been validated to detect driving-ability impairment. A fundamental issue in this area is that the word ‘impairment’ is used extensively but not consistently in the scientific research and the legislation. It may refer to driving-ability impairment. It may refer to drug influence, which is impairment of the ability to perform tests designed to look for a drug. It may refer to physiological signs associated with drug use or to a specific blood-alcohol or drug concentration. In the UK, section 5a of the Road Traffic Act 1988 sets per se limits for several drugs. Section 4 of the Road Traffic Act is an ‘impairment’ offence. It is an offence ‘to drive while unfit to do so through drink or drugs’, and a person is taken to be unfit if their ‘ability to drive properly is for the time being impaired’. To be unfit to drive, a person’s ability to drive must be impaired, and there must be a causal link between the impairment and any drugs found in their system. Similar laws exist around the world, pairing per se limit offences and impaired driving offences. To assist in gathering evidence of driving-ability impairment, section 6B of the Act gives police the power to administer preliminary impairment tests. These tests, a pupillary examination and four divided attention psychophysical tests – the modified Romberg balance test, the walk-and-turn test, the one-leg stand test and the finger-to-nose test – are derived from the more extensive Drug Evaluation and Classification (DEC) programme from the USA. Trained health-care professionals are also permitted to assess drivers using these impairment tests. However, following a reappraisal in June 2019, the Faculty of Forensic and Legal Medicine withdrew its support for the tests, stating on their website, ‘The Field Impairment Tests (FIT) have never been scientifically or statistically calibrated using a control group of subject drivers who had not taken any drugs’. The UK’s Preliminary Impairment Tests are derived from the American DEC system, itself an extension of the Standardized Field Sobriety Test (SFST) system, neither of which are designed or validated to detect driving-ability impairment. The SFSTs were originally developed as a predictor of blood alcohol concentration (BAC). The authors of the SFSTs are unequivocal that the system does not detect driving-ability impairment. The SFSTs detect alcohol influence on a subject’s ability to perform those tests, such that the operator can then estimate the subject’s BAC. In a SFST study from 1998, it was noted that: