Reports indicate that in Victoria, New South Wales and Western Australia, 23.5% of drivers in fatal accidents had consumed drugs other than alcohol, and that 29.1% of drivers had a Blood alcohol concentration (BAC) level of 0.05% or higher. Alcohol has been detected in combination with drugs in almost 10% of cases. Cannabis was most prevalent among drugs other than alcohol detected in specimens (13.5%). The combination of drugs as an influence on road traffic accidents is becoming a growing concern and research has been conducted to identify how these drugs impair performance. In Victoria, Standardised Field Sobriety Tests (SFSTs) have been introduced as means of testing for impairment in drivers who have consumed drugs other than alcohol. The use of SFSTs, although designed for the detection of alcohol-intoxicated drivers (up to 0.08%), has been implemented in programs for the detection of drugs other than alcohol. The present study had several aims: to examine the effects of cannabis and cannabis together with alcohol on driving performance; to examine the effects of cannabis and alcohol on SFSTs performance; to examine the efficiency of SFSTs to predict driving performance associated with the administration of cannabis and alcohol; to examine any differences between the effects of cannabis and alcohol on performance in regular cannabis users and non-regular cannabis users; and to examine any differences between SFSTs ratings by researchers and SFSTs ratings by police officers in order to identify the inter-rater reliability of SFSTs. (Executive summary, edited)