Drivers whose judgement has been impaired pose a risk not only to themselves but to all road users. With the legalisation of marijuana for personal consumption in South Africa, how much is too much before you shouldn’t be allowed to drive?

“Drug driving is certainly not a new problem, but the legalisation of marijuana has seen a definite increase in the amount of cannabis being consumed,” notes Rhys Evans, managing director of ALCO-Safe. “It is therefore likely that there has also been a concurrent increase in the number of people driving under its influence.”

He continues: “Tetrahydrocannabinol (THC), the psychoactive component of marijuana, causes impaired judgement and slowed reaction times, much like alcohol does. It can also cause or worsen anxiety and depression, amongst other side effects. Consumption of marijuana, therefore, has a negative impact on a person’s ability to drive.”

A report commissioned by the United Kingdom’s department of transport reveals that the “risk of a road traffic collision whilst driving under the influence of cannabis was significant and almost twice the risk compared to driving having not consumed this drug”.

The report, which was conducted by the European Union’s research project on Driving under the Influence of Drugs, Alcohol and Medicine (known as the DRUID project), also highlights a connection between higher concentrations of THC in the blood and fatal accidents. Higher consumption, or more recent consumption, seems to be related to a higher risk of fatality.

“On their own, both alcohol and marijuana make driving hazardous,” Evans says. “When they are mixed, the effects are amplified. If alcohol is consumed before marijuana, the absorption of THC is increased, which magnifies its effects. Driving under the influence of either is dangerous, but the combination of the two could easily prove to be deadly. The DRUID study shows that the possibility of having an accident increases 16 times more than that of a sober person.”

One of the challenges, however, is that there is currently no accepted method in South Africa for testing on the roadside. There is also no set limit for what is considered to be “under the influence”. The effects of THC are similar to alcohol in that they depend on the strength of the drug and the metabolism of the individual, as well as how frequently they consume it.

“Studies have been conducted across the globe to assess the level at which impairment is likely, and this information can be used to ‘draw a line in the sand’ as to what the legal limit is,” Evans adds. “Government needs to engage with industry experts and drug testing companies to develop appropriate policies and methods for curbing this growing problem.”

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