Background: Drug use among the driving population is a major hazard to road safety and has been the subject of widespread research worldwide. In an attempt to detect and appropriately prosecute “drug-drivers”, the UK has made legislative changes and has introduced Field Impairment Tests (FIT) in the Railway and Transport Safety Act 2003. These FIT, which are identical to the Standardised Field Sobriety Tests (SFST), were devised in the USA in the 1970s to identify clinical signs of impairment due to alcohol intoxication, but were not designed or intended to identify drug-related driving impairment. Concerns have been expressed that FIT are too difficult for their stated function, and are also inappropriate tests, since although they have been validated for alcohol effects they have never been validated for the effects of drugs. This thesis has sought to clarify matters and has questioned the validity of FIT by testing two opposing hypotheses – 1) FIT are reliable and valid tests of drug-related impairment to drive - and drug-free individuals perform well on all tests. 2) FIT are not reliable and valid tests of drug-related impairment to drive - and are too difficult for some groups of drug-free individuals to perform. Methods: A questionnaire was designed and a postal survey was undertaken of 960 Forensic Medical Examiners (FMEs) who were asked to give their opinion on the Field Impairment Tests (FIT). The responses of the FMEs were analysed in detail (chapter 3). FIT were then carried out on three separate groups of 100 subjects in police custody who were all known to have used no drugs for a period of at least 8 hours prior to the testing process (chapter 4). Group A subjects were opiate dependent; Group B were subjects who received legally prescribed methadone; Group C individuals denied any form of drug use. All study groups were simultaneously examined using conventional psychomotor tests, and the results were compared using detailed statistical analysis with logistic regression and summative scores. Results: Returned FME questionnaires showed 63% of FMEs considered the tests “about right” but a significant number (p<0.0001) of 33% of FMEs stated FIT were “too difficult”. The studies on subjects in custody clearly showed 82% of group A; 44% of group B; and 19% of group C were unable to complete FIT satisfactorily, although only 12% of group A; 2% of group B; and 3% of group C were unable to successfully perform conventional psychomotor and cognitive tests. These findings clearly supported the assertions held by 33% of FMEs surveyed that FIT were too difficult for their stated purpose and that poor performance in FIT could not be regarded as definitive evidence of drug-related impairment in driving ability. VII Conclusions: The results of the research studies conducted have provided very strong support in favour of hypothesis 2) FIT are not reliable and valid tests of drug-related impairment to drive - and are too difficult for some groups of drug-free individuals to perform. In an attempt to overcome the problems in respect of FIT, specific proposals have been offered including the introduction of a new battery of more relevant clinical tests of impairment (CTI); a change in the method and manner in which the proposed new tests are applied; and possible legislative and administrative measures which might be introduced to more appropriately and effectively tackle this on-going hazard to road safety.

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