"To combat drug-driving, most countries either operate a zero tolerance policy or take into account degree of impairment, sometimes in a two-tier system. Legal limits may be set low, at the limit of detection, or higher to take effects into consideration. For example, while the project set a detection limit of 1 ng/ml in whole blood for THC in the roadside surveys, it was found that 2 ng/ml THC in whole blood (3.8 ng/ml THC in serum) seems to cause impairment equivalent to 0.5 g/l BAC. Such equivalents could not be calculated for other drugs. It is not realistic to develop cut-off limits for all substances."
European Monitoring Centre for Drugs and Drug Addiction, "Driving Under the Influence of Drugs, Alcohol and Medicines in Europe — findings from the DRUID project" (Luxembourg: Publications Office of the European Union, 2012), doi: 10.2810/74023, p. 7.