The Police Federation have called for the drink-driving limit to be lowered in England and Wales, to bring it in line with Scotland and most other parts of Europe. They have claimed that people, in particular women, are not heeding the current limit, and putting lives at risk.
What is the scientific evidence for the current and proposed limits? And just how much alcohol do you need to drink before your driving will be impaired?
Alcohol has a negative impact on a variety of the skills needed for driving. Not only does it slow our reaction times, but it can impair our judgement too, which might make us more likely to get behind the wheel when we’ve had one or two too many. This is why we ourselves might not be best placed to judge whether we’re safe to drive or not, and emphasizes the need for legal limits.
But the current limits are somewhat hard to interpret. It’s hard enough to work out how many drinks correspond to a unit, let alone how that translates to blood alcohol volume! The current limit in England and Wales is 80mg of alcohol per 100ml of blood. In 2014, Scotland lowered their limit to 50mg per 100ml.
How much alcohol gets in to your blood from one drink depends on all sorts of things, from when you last ate, to your weight, sex, height, metabolism and even current stress levels. So even if you ‘know your limits’ this might not help you judge it, as the day (or meal) you’ve had can affect how alcohol will be absorbed in to your blood.
Alcohol depresses neural activity. That is, alcohol slows your brain down. There’s plenty of evidence that even low doses of alcohol can have effects on reaction time and attention. These effects can be somewhat small, but when you’re in charge of a car going at speed, small impairments are magnified.
A review of the literature on alcohol-related driving impairment by the USA’s Department for Transportation in 2000 compiled evidence of alcohol’s effect on various functions, including cognition, drowsiness, psychomotor skills, reaction time and driving simulator performance. Even at alcohol levels well below the current guidelines, there was some evidence of impairment in all of these areas. That said, other research has shown that not all of these different measures are affected at exactly the same levels of alcohol consumption, meaning it’s very difficult to determine what is a ‘safe’ level of alcohol use for the skills needed for driving.
But none of this is new evidence, so why the push to change the limit now? The Police Federation report that lowering the limit in Scotland has led to a reduction in failed breathalyser tests, but it’s probably too early to tell whether this is a true reduction or random variation. The Government have considered lowering the limit before, and even commissioned a report in 2010, which recommended the reduction take place, but the Government at the time concluded that better enforcement of the current law would be more effective.
Given that it’s already hard enough to work out whether you’re over the limit or not, it could be argued that a reduction in the limit could add confusion, as some people would need to drink less than a pint or small glass of wine to put them over it. But maybe this would this mean people might avoid drinking altogether so as not to risk being over the limit?
Some European countries actually ban drivers from consuming any alcohol at all. Is this something England and Wales should consider? Not according to the British Medical Association. They supported Scotland reducing the limit from 80 to 50mg per 100ml of blood, but categorically state they are against a zero-level limit, pointing out that sometimes a person will give a non-zero reading even without drinking, for example if they have recently used mouthwash.
As an aside, it’s also worth noting that although the Police Federation are worried about the message not getting through to women drivers, the vast majority of convictions for drink-driving are still for men (45,471 compared to 9,586 in 2012, according to Ministry of Justice statistics).
If the Police Federation are correct, and the message about the dangers of drink driving isn’t getting through to drivers, then the scientific evidence seems to support the idea that a reduction in the limit would lead to fewer accidents, as it seems that the more alcohol you drink, the more you are impaired. As NICE’s guidelines state “studies consistently demonstrate that the risk of having an accident increases exponentially as more alcohol is consumed”.
Perhaps just as importantly, though, better education and explanation as to both the impairments alcohol can cause, and the real-life drinks that can be consumed before a person is over the limit would help people to ensure that they stay under the limit and don’t put their own or others’ lives in danger.
Thanks to Dr Sally Adams for her comments on a draft of this article.
