||The examples and perspective in this article may not represent a worldwide view of the subject. (December 2010)|
Several American states and European countries now have "per se" DUID laws that presume a driver is impaired if they are found to have any detectable quantity of controlled substances in their body while operating an automobile and that the driver has no doctor's prescription for the substance. This is similar to the "per se" DUI/DWI laws that presume a driver is impaired when their blood alcohol content is above a certain level (currently 0.08% in the United States). There is some controversy with "per se" DUID laws in that a driver with any detectable quantity of controlled substances may not in fact be impaired and the detectable quantity in blood or sweat may be only the remnants of drug use in days or weeks past.
This controversy is reduced by reducing the potential for wrongful convictions in 2 ways; firstly if blood tests are used to obtain convictions most legal frameworks require corroborating evidence of impairment to support that the blood test result represents impairment. This ranges from suspects being required to undertake physical co-ordination tests, to computerised pupil reaction tests, to use of witness accounts of general behaviour and erratic driving, and may include expert testimony as to expected impacts on performance.
Secondly, when saliva tests are used (Australia), a cut off threshold is calibrated in fitting with U.S. SAHMSA standards, at such a level that the average person would be impaired by the drug detected to a degree that crash risk is significantly raised. Some U.S. States have blood limits for illegal drugs which are merely indicative of use, and studies have shown that this use can be as long as weeks in the past (see: 7 Nev. L.J. 570 (2006-2007) DWI and Drugs: A Look at Per Se Laws for Marijuana). In Germany where more economical sweat tests are used, it is not possible to calibrate detection to a level linked to impairment, and all users may be identified. So unless there has been injury caused there is no criminal charge, but only an administrative one of driving with drugs present, which does not infer impairment or lack of it. This is similar to a speeding ticket as a disincentive for potentially risky behaviour. However if injury was caused, a process of further evidence gathering of driver impairment may be triggered, which can lead to a criminal violation.
Systems where there is a penalty after detection of target drugs are sometimes misnamed zero tolerance for the foregoing reasons. Genuine zero tolerance jurisdictions (ones not applying rational cut offs) are generally failing to substantially reducing drugged driving, where it has been a problem originally. However, in Australia where the use of calibrated devices create de facto limits for common impairing illicit substances, roadside detections and self reporting of drug driving in National telephone surveys have reduced by 25% over the last 3–4 years.
Varying DUID laws have been passed over the last 15 years in response to both increasing roles of drug driving in road tolls, and the fact prosecutors have found it difficult to prove that a driver was impaired from using a controlled substance. Practical difficulties included the transient effects of some drugs wearing off before either police or doctors had a chance to assess many suspects for impairment, and also the expense of having to call expert forensic witnesses before courts to interpret results on a case by case basis. These laws can make their cases much easier to win if they only have to prove the presence of a controlled substance in the blood or urine, without a prescription. The logic in zero tolerance jurisdictions is that the trade off more efficient prosecutions of potentially impaired individuals is well worth the possible erroneous convictions of a lesser number of drivers who may not in fact be impaired, because the driver was already violating the law by using a controlled substance without a prescription.
There is a tension with such an approach and enhanced road safety. Promoting zero drug use, as with alcohol prohibition, is not a realistic goal and Policing and Court resources are clearly best targeted at hazardous use. The most successful laws for reducing drink driving have sought to modify cultural norms and the target groups behaviour, separating impairment and driving. An approach that appears to be working most successfully to reduce drug driving, based on the Australian experience of matching public health guidelines, such as no driving for approximately 3 hours after cannabis use, with targeted legal intervention (the saliva test drug detection thresh ehold).
New Zealand Laws
New Zealand in 2009 passed unique legislation, which created a zero tolerance law for driving under the influence of class A drugs (methamphetamine) to discourage their use, and a requirement for impairment testing if other drugs are suspected. New Zealand's main issue in traffic is Class C drugs, so this law could appear oriented to support the "war on (hard) drugs". Road Safety advocates however supported the provision for intolerance of methamphetamine, introduced by Opposition MP Trevor Mallard. As intoxicated users can pass traditional impairment tests despite being unfit to drive (mood and judgment), and the time that drivers may be most at risk is when in the come down and levels are low, making any cut off level in future self-defeating.
Drug impaired driving is today suspected by the European Road Safety Observatory of being the reason that drink driving crash rates no longer reduce in direct proportion to reducing or plateaued numbers of drunk drivers found on roads. It is believed that the remaining ones are carrying more risk than their blood alcohol levels should strictly impart - due to the frequent addition of other drugs. If the historic assumptions about formulae to setting alcohol limits at particular levels to reduce harm (by anticipated degrees) in the target demographic are defunct, the implications for impaired driving reduction policy are major.
United Kingdom Laws
Recent UK legislation effective 2nd March 2015 has introduced a series of new offences into the realm of road traffic law.  The government has sought to tackle the belated increased number of drivers operating vehicles with a high measure of proscribed and prescribed drugs in their system. The new offences detail at least 8 illegal drugs and 8 legally prescribed drugs, which have been given set levels to be tested. Illegal drugs will have a 'trace' level set that would constitute a criminal offence.
Drivers who are taking legally prescribed medication will be required to closely follow their doctor's and drug manufacturer's guidance when taking the medication. If the driver is found to be carrying more than the set levels in their system, outside of the set guidelines, they will be guilty of an offence under the new rules. 
- "Drug drive legislation: am I fit to drive?". GOV.uk.
- Sunil Rupasinha. "New Drug Drive Legislation: 2 March 2015, with Sunil Rupasinha".