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THC vs Alcohol Impaired Driving

A police car pulling over a drunk driver

By Dr. Shanna Marrinan

Since the 2nd of March 2015, it has been illegal to drive with predetermined amounts of particular drugs (including legal medications) in your blood. The limits for these legal and illegal drugs can be located in ‘The Drug Driving (Specified Limits) (England and Wales) Regulations 2014’ (hereafter the 2014 regulations’). However, a handy guide can be found on the Release website.

Simultaneously, it is a criminal offence to drive a vehicle whilst intoxicated if that intoxication impairs the driver’s ability to drive properly.

The primary difference between the two aforementioned offences is that under the 2014 regulations, an individual can be found guilty of an offence regardless of actual impairment as this offence relies on traces of specific drugs in a person’s blood. The limit for some of these illicit drugs has been set very low: just a small amount of a controlled substance in the blood will result in the user being over the limit.

One of the drugs contained in the 2014 regulations is Delta-9-tetrahydrocannabinol (THC), which has a limit of just 0.2 microgrammes per 100 millilitres of blood. The detectability ‘window’ for THC is comparatively much longer than for many other substances, meaning analytes can sometimes be detected in biological matrixes (particularly in urine) well after all psychological and physical effects have finished. This is problematic, as Release highlight “because of the very low-level set, frequent/heavy users will have a store in their system that can re-release so may test positive after many days even if they haven’t used”. The level was set much lower than that recommended the government’s own commissioned report, which suggested a level of 0.5 microgrammes /100mls would be sufficient for non-impaired driving. It is believed that this was an attempt to deter recreational cannabis use rather than to make driving safer.

This becomes increasingly worrying as new research demonstrates disparities of culpability contributed by both cannabis and alcohol towards driving-related injuries. A recently released Australian study conducted a culpability analysis of 5000 driving injuries, in which a drug test was conducted.

Drivers with alcohol present showed large increases in the odds of culpability similar to that seen in other studies investigating associations between blood alcohol concentration and crash risk.

Drivers with THC identified in their system were found to have a drug culpability score of 1.9 times that of the control group (sober drivers). Comparatively, drivers with alcohol in their system had a culpability score that was 16 times higher than the control group’s.

Additionally, the grouping of those that had THC in their system was split into four separate groups.

  1. 0.1 – 0.49 microgrammes per 100 millilitres of blood.

  2. 0.5 – 0.99 microgrammes per 100 millilitres of blood.

  3. ≥0.5 microgrammes per 100 millilitres of blood.

  4. ≥1 microgramme per 100 millilitres of blood.

It should be noted that the UK limit is set at 0.2 microgrammes per 100 millilitres of blood. Therefore, the lowest grouping in this study would encapsulate a lot of individuals that would be over the limit whilst driving in England and Wales.

The increase in odds was most apparent at higher blood THC concentrations. The lowest grouping (0.1-0.49 microgrammes per 100 ml of blood) had a culpability rating of 1.6 times of the control group. Whereas the highest grouping’s (≥1 microgramme per 100 ml of blood) culpability rating was 10 times that of the control group. Only 1% of the study’s participants fell into this high THC group.

In contrast, here are the groupings for alcohol:

  1. 10 – 49 milligrammes per 100 millilitres of blood.

  2. 50 – 99 milligrammes per 100 millilitres of blood.

  3. 100 – 149 milligrammes per 100 millilitres of blood.

  4. ≥150 milligrammes per 100 millilitres of blood.

The blood alcohol concentration (BAC) limit for England and Wales falls into the second grouping (50-99 milligrammes per 100 ml of blood), this group had a culpability rating of 5.7 that of the control group (3.5 times higher than the THC group). Unsurprisingly the more alcohol consumed the more culpable it was as a factor in the collision. In the highest grouping in the alcohol category (≥150 milligrammes per 100 ml of blood), alcohol had a culpability rating that was a shocking 73 times higher than the control group!

The impact of THC on driving capabilities is recognised as moderately negative. This needs to be contrasted, however, against the far larger impact of alcohol consumption on driving safety. In the UK, we have a comparatively lenient blood-alcohol limit compared to many other countries, including most parts of Europe, where the limit is generally 50mg (as opposed to the UK’s 80mg) per 100 ml of blood. Despite this, 41% of those drivers testing positive in the UK in 2017 had BAC at more than twice the legal limit. Given that significant impairment has been documented at the far lower BACs (20mg/100ml) this is very concerning indeed.

In addition to the impact of alcohol consumption on tangible abilities such as perceptual and motor functioning, there is also likely a large impact on ‘emotional aspects of behaviour, such as judgement, aggression and risk-taking’. This is, of course, particularly pertinent at this time of year (with higher-than-usual numbers of people engaging in ‘drink driving’ over the Christmas period). Deaths from collisions where at least one party is over the legal blood-alcohol concentration appear to be on the rise. The most recent Department for Transport figures cover the period up to 2017 (subsequent data is due for release in Feb 2020) reveal that there were 250 deaths in 2017 which is the highest number of fatalities since 2009. A total of 8600 of alcohol-implicated casualties (crashes resulting in either injury or death) were recorded that year. Meanwhile, while there were only 74 recorded collisions in total involving drivers testing positive for other drugs (including cannabis).

Despite evidence that demonstrates that alcohol impairs drivers far more than THC, the limits for THC are far lower than that of alcohol. It should be noted that THC does impair an individual’s ability to drive a vehicle. However, the current limit is so low that it will criminalise a lot of people who are not affected by the THC they have consumed, at the time of driving.

Another important point to make is that the Australian study considers only those drivers who have already been involved in a collision, to assess culpability. The likelihood of being involved in a collision in the first place is not directly addressed. Previous studies have determined that the impact of psychoactive substances on driving ability and safety is complex; THC, whilst having a negative effect on skills such as distance perception and speed of response, also discourages risk-taking. Drivers may drive more slowly, and be less inclined to engage in risk-taking behaviours than those who have not ingested THC. Conversely, alcohol may inhibit not only reaction times, spatial awareness and distance perception, but may also increase the likelihood of the user engaging in risky behaviours and impair decision-making. In practice, this may mean that despite the recognition of a limited amount of THC-induced impairment in technical driving ability, the overall likelihood of having a crash is partially mitigated by the tendency to drive more slowly. Although a number of studies have identified a somewhat elevated for those with a detectable presence of THC, A 2009 study found cannabis users were also more likely to successfully compensate for impairment using a variety of strategies such as maintaining a longer distance between themselves and car in front. Furthermore, impairment due to THC rapidly declines with regular use; naïve users are much likely to experience a negative impact on driving skills vs daily consumers who have acquired a tolerance. While both driver impairment and likelihood to crash are both positively correlated with THC consumption, the risk is far lower than that posed by ‘drink driving’, a statement supported by the findings of the Australian study, which identifies a very modest rise in odds of culpability for the group who had detectable levels of THC, against large increases in drivers who presented with alcohol concentrations in their blood toxicology.

In attempting to predict the impact of a broadening of access to medical cannabis, or any future change to laws around recreation use, we can look to those US States which have already been through these processes. Research into the impact on road traffic accidents in parts of the US which have legalised cannabis found a small and temporary spike in road fatalities (one additional death per million inhabitants), followed by a return to baseline, suggesting an initial increase in cannabis consumption amongst inexperienced users, who were comparatively more likely to experience driving-related impairment. This appeared to ‘level off’ once States adjusted to having access to medicinal or recreational cannabis.

When we think about cannabis-based medical products, which will, in most cases, be used on a daily basis, the chances of a positive detection rise significantly. It is not clear whether any significant risk is posed to driving with regular, moderate consumption, beyond perhaps the initial adjustment period. Luckily, this will be taken into account by law enforcement due to S.5A(3)b of the Road Traffic Act 1988 which identifies a defence for anyone who has taken a specified “drug in accordance with any directions given by the person by whom the drug was prescribed”.

In conclusion, this new study presents further evidence that Delta-9-tetrahydrocannabinol has relatively little impact on driving especially compared with alcohol. The current limit is unfairly penalising those who may have detectable levels of THC present, but be well outside of any period of impairment. When this is contrasted with the levels set for alcohol, which is known to induce significant functional impairment at much lower levels and which has a much shorter detectability window, the potential for this policy to unnecessarily criminalise cannabis-consumers is large. At the same time, currently, policy fails to adequately address the risk posed by alcohol users. As such, we believe this policy should be revised to appropriately consider the level of risk posed by each substance respectively.

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