Legislation in all Australian states and territories creates offences and provides for police roadside testing in relation to ‘drug driving’. Ostensibly motivated by the same road safety objectives and impairment paradigm as drink driving laws, drug driving laws adopt a significantly different approach. Whereas random breath testing tests for all forms of alcohol and is designed to determine whether there is a sufficient concentration of alcohol in the driver’s body that s/he should be deemed to be impaired, random drug testing typically tests for the presence of any quantity of only the three most widely used illicit drugs—cannabis, methamphetamine and ecstasy—in the driver’s oral fluids, without reference to what is known about the different pharmacokinetic and pharmacodynamic qualities of different drugs. This article examines this idiosyncratic approach to the criminalisation of drug driving, highlighting its weak correlation with the important road safety objective of deterring substance-impaired driving, and the risks of both over- and under-criminalisation that it creates. It argues that public policy on the prohibition of certain drugs and the criminalisation of their use should be disentangled from public policy on impaired driving. It recommends that drug driving laws in all Australian jurisdictions should be brought back into line with drink driving laws, via legislation and testing practices that turn on substance-specific prescribed concentrations for all drugs (illicit and licit) that have the potential to impair drivers.