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Annual 'cannabis day' associated with increase in car accidents in the UK

An annual day that celebrates cannabis intake may contribute to over 20 additional car crashes involving in injury or death in the UK, finds research from King’s Business School and Harvard University.

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The Department of Transport estimates that car crashes cost the UK economy £35 billion* per year, and that the average cost of an injury-causing accident is £90,000. Identifying factors that may increase the number of car accidents so that preventative measures can be developed could result in cost-savings, as well as reducing social harm. 

While some studies have found that the risk of vehicle collisions increases after cannabis use, evidence to date has been inconsistent and direct links between cannabis intake and car accidents have often been difficult to make. This is especially the case in UK where the drug is illegal. But researchers now suggest that the rise in popularity of ‘World Weed Day,’ which takes place from 4.20pm on the 20th of April each year, may provide a good opportunity to study this across Britain. 

In a paper published in Accident Analysis & Prevention, Dr Sotiris Vandoros, Senior Lecturer in Health Economics at King’s Business School and Dr Ichiro Kawachi, Professor at Harvard University, looked at data on motor vehicle collisions that involved injury or death reported by all 51 police forces across Britain between 2011 and 2015.

The team focused on accidents that occurred after 4.20pm on the 20th April and compared this to accidents from 4.20pm onwards on the same day of the week in the week before and two weeks after the 20th. 

They found that there were 17.9% more crashes – equating to 23 – after 4.20pm on the 20th April than at the same time on the other days they studied. This was even after controlling for factors such as unemployment rates and high petrol prices that lead to less people on the road. By looking at records from previous years, they noted that the relative number of crashes on the 20th April appeared to have increased in the UK only after cannabis celebration day became more widespread. 

Over this period, the day also demonstrated a higher relative increase in accidents than over 96% of days across the whole year, when they were compared to days in the preceding week and succeeding two weeks. 

The work suggests that this increase in accidents might be linked with cannabis use, but other factors on that day may also play a role, such as having younger drivers on the roads, risk-taking behaviour, or alcohol consumption.

Dr Vandoros says: ‘An increase in accidents can have a large emotional and monetary cost for the UK and so if potential pressure points can be identified, these can help raise driver awareness and potentially avoid accidents on high risk days in Britain.’

‘Further research now needs to be carried out in order to develop methods to try and reduce accidents on these days.’

*Figures from the UK Department for Transport