Would 50 mg Work?

What would be the effect in practice of a 50 mg drink-driving limit?

While there has been extensive debate about the merits of reducing the drink-drive limit and the potential impact on pubs, there has been little discussion of what effect it would actually have on people's real-world behaviour. While I have clearly expressed my scepticism about a limit reduction elsewhere on this site, my objective here is not to argue the case against it but to try to evaluate the practical consequences.

There are basically three categories of drivers who may have a blood alcohol level (BAC) between 50 mg and 80 mg and whose behaviour might be changed by a limit reduction:

  1. High-Risk Offenders: those who at other times are prepared to drive with a BAC well above 80 mg but by chance happen to be below it at this particular time
  2. Morning After Drivers: Those who are unaware of their alcohol level because it has arisen from drinking many hours before or on the previous day
  3. Two Pint Men: Those who are aware of the limit and are trying to keep below it
The article then examines the question of Police Enforcement before reaching its Conclusions.

The High-Risk Offenders

This group, broadly speaking, comprises those who are driving with a BAC over twice the 80 mg limit, a level at which the risk of a fatal or serious accident as at least twenty times that with a zero BAC. They account for half of all drink-drive convictions and are responsible for around 85% of all casualties attributed to excess alcohol. Inevitably, as their alcohol levels rise or fall, there must be times when these people are driving with a BAC between 50 and 80 mg and they therefore make up a substantial section of drivers in this range.

However, it is hard to see that it would make any difference to their behaviour. If they are quite prepared to exceed 80 mg by a wide margin, then they would exceed a lower limit in just the same way. At these levels, nobody can be under any illusions that their behaviour is way outside what the law allows, and, unless they are drinking alone, neither will their companions. Indeed the government's own consultation paper assumed that a lower limit alone would have no effect whatsoever on these high-risk offenders.

The only way a lower limit could make a difference is if it created a climate where the high-risk offenders were deterred to a greater degree than at present because their behaviour stood out more clearly. But even that would probably only have a very marginal effect, because, as stated before, these people will either have been drinking alone, or in company that is prepared to tolerate gross offending.

The consultation paper also mentioned what is potentially a separate category - drivers with BAC levels between 80 and 110 mg who it suggested might be making some attempt to adhere to the limit but misjudging it, and therefore might be encouraged to modify their behaviour. In reality, I do not believe this is the case. The conventional wisdom that the 80 mg limit is equivalent to two pints in fact understates it somewhat - in practice, two pints of 3.5% ABV beer would give a man of average weight a maximum BAC of about 60 mg. Given this, the driver with a BAC of 110 mg is probably (or should be) well aware that what he is doing is likely to take him above the limit, and may well have consumed three and a half or four pints of normal strength, or two and a half pints of 5% beer. I would therefore suggest that these are either drivers who in effect "set their own limits" and feel unlikely to be caught, or high-risk offenders who just happen to fall in the 80 - 120 range on this occasion. It is therefore unlikely that the lower limit would have much effect on the behaviour of drivers in this range.

The Morning After Drivers

Over the years, the "morning after" issue has been downplayed by the government because it was seen as blurring the overall message about the dangers of drink-driving, and because to understand it requires a knowledge of "unit counting" which could also be used to "drink up to the limit". But, with a lower limit, people would end up in this category after what to many is a normal evening in the pub rather than a "heavy session". Drivers should not be left in ignorance and exposed to a risk of breaking the law which they might well avoid if they were better informed. Anything more than four pints of normal strength, or three pints of 5% beer, could put someone over a 50 mg limit the following morning.

This is not widely understood at present, and I have come across a number of people, both in CAMRA and outside, who have been ostentatiously unwilling to drink as much as a half-pint immediately before driving, but who must on occasions have driven when well over the limit due to heavy drinking the night before. Again, this suggests that a lower limit would make little difference to the behaviour of morning after drivers unless the implications were properly appreciated.

The whole subject of "morning after" driving is one on which the government's consultation paper was strangely silent, yet I would suggest that even now it probably accounts for a majority of the technical drink-driving offences that occur, as opposed to those that are detected by the police. Motoring organisations have expressed the view that most of the additional prosecutions under a lower limit would fall into this category.

To have an impact on behaviour it would be necessary to mount a high-profile publicity campaign pointing out very clearly how slowly the body metabolises alcohol, and the risks of still being over the limit the following morning after what many would see as a tame evening's drinking. This has the potential to make a lot of people think seriously about their behaviour, as even the people who are currently driving "the morning after" when well over 80 mg would in many cases be fundamentally law-abiding individuals who do simply not realise the implications of their actions. Inevitably such a campaign would have to involve the dreaded "unit counting", as it is clearly unreasonable to insist that people should have nothing to drink the evening before driving (although I have seen this seriously suggested by supposedly reputable people from road safety groups). This objection could be met if the campaign concentrated on the complete elimination of alcohol from the bloodstream rather than its reduction below 50 mg.

However, there is considerable evidence to suggest that, for drivers in the "morning after" category, the risks may be overstated, as the effect of alcohol on impairing driving performance tends to wear off more quickly than the actual level of alcohol in the bloodstream. This is a recognised medical phenomenon known as the "Mellanby effect". Therefore, even if a substantial change in behaviour was achieved, it is doubtful whether it would in practice have a significant impact on casualties.

With a 50 mg limit, there would be a growing demand for self-test breathalysers to ensure that drivers did not fall foul of the law on "the morning after". In the past there has always been an objection to these on the grounds that they encouraged people to drink up to the limit, and while they have been available they have not been widespread. This objection would obviously carry much less weight with a 50 mg limit, and surely could be completely overcome if the devices were specifically targeted at morning after drivers and calibrated to give a positive reading over 20 mg (the Swedish legal limit) rather than 50 mg.

The Two-Pint Men

These are the people (and they are usually men) who are visiting licensed premises and drinking an amount of alcohol that they believe will put them in the 50 - 80 mg BAC range. Two pints of ordinary strength beer is the commonly accepted figure of the maximum you can drink and still stay below 80 mg, but they may in fact be drinking one and a half pints, or two and a half, or even three, and indeed they may be deluding themselves. The key factor is that they believe they are staying within the law.

This is the group where a lower limit would have most effect, because these are people who broadly speaking want to obey the law. Probably a majority would change their behaviour and on most occasions ensure they drink no more than whatever amount they believe will keep them within 50 mg. Whether they did this by going to the same pub and drinking less, not going to the pub at all, or leaving the car at home and walking to another pub, is largely irrelevant here.

That may well have a major impact on the business of pubs in out-of-town locations. But it is much less clear that it would make much difference to casualties. In practice, because of the extremely unpredictable rate at which alcohol is absorbed by the body, nobody can "drink up to the limit". Either you play safe, and stay comfortably below it, or you try to drink up to the limit, and run a serious risk of exceeding it.

So the people who are trying to abide by the law are general at the lower end of the 50 - 80 mg range, where the additional risk is small or non-existent. As mentioned above, two pints of beer in the 3.5% - 4% ABV range will probably lead to a maximum BAC concentration of around 60 mg in an average sized man, and that will not occur until about an hour after he has finished his second pint, by which time he may well have driven home from the pub. The "two pint man" may in reality not even exceed 50 mg while driving. That of course is no guarantee that he won't, but it does indicate clearly that even if you succeeded in getting most of them to cut down to one or one and a half pints, any reduction in casualties would be marginal in the extreme and would be lost in the noise of accident statistics.

The government consultation document came up with some figures (without properly identifying the source) that indicated that the additional serious accident risk in the 50 - 80 mg range could be up to 5 times that of a completely sober driver. However, my interpretation of the Grand Rapids (Borkenstein) study, which is the main source of these statistics, is that the additional risk is more in the order of 20 - 50%, and half as much again as negligible is still pretty much negligible.

It has been suggested that any new legislation might not give driving bans to people convicted of driving with alcohol levels between 50 and 80 mg, but merely impose fines and penalty points similar to speeding offences. If this were to be the case, it is likely that many who were previously in the habit of driving after "two pints" would continue to do as they did before, at least until they were caught for the first time. These people have already ignored the official advice not to touch a drop before driving and presumably are not in agreement with the limit cut, so they could well be inclined to chance their arm.

In practice you are still very unlikely to be breath tested unless involved in an accident, particularly at lunchtime, and many would probably ride their luck for a long time. (I don't propose to turn this into an examination of my own behaviour, but suffice to say I have been driving since 1976 and have been breath tested precisely once). Indeed in the past few years, the increasing replacement of traffic patrols by speed cameras has reduced the chances of being tested. Some might cut down for a while (maybe from two or two and a half pints to one and a half) but, if they escaped being stopped and tested for a few months, be tempted to revert to their previous behaviour. This further undermines the argument that an assault on the habits of the "two pint men" would make much difference to road casualties.

The other penalty option that has been mooted is to impose a shorter driving ban, say of six months, on drivers in the 50 - 80 mg range. This obviously would seem disproportionately harsh to many, and could even encourage some who previously did their best to keep below 80 mg to take the view that they might as well be hanged for a sheep as a lamb. A ban is a ban, and someone banned for six months would have to make all the same inconvenient lifestyle adjustments as they would if banned for a full year, albeit for a shorter period. The risk of losing their job would probably be just as great. While mandatory bans for 50 - 80 mg offenders would undoubtedly encourage more "two pint men" to drop down below 50 mg, they would provide no meaningful incentive to keep below 80 mg, and could lead to an increase in people knowingly driving in the 80 - 120 mg range.

Police Enforcement

There is a question mark as to whether the police would enforce "blitzes" on pubs to try to catch people driving in the 50-80 mg range, if they felt that a lot of pubgoers were chancing their arm, as mentioned above. If they did, it would breed a great deal of resentment, and they could be accused of trying to put pubs out of business. Even if you didn't drink anything alcoholic you would still get pretty fed up being repeatedly breath tested when driving away from pubs and it might make you think twice about visiting them at all. However, the police could well see this as counter-productive in terms of their limited resources (although that does not stop them indulging in other forms of pointless behaviour).

There is also an inherent impracticality about the "blanket testing" of drivers leaving a particular pub as, as soon as one was tested, the message would get back to the other customers in the pub. Any who felt they may be running a risk would then stay put until the danger is passed, or get someone else to drive them home. While this may not be an effective method of catching drink-drive offenders, it could be a very good way of ruining individual pubs. The survival of country pubs in any particular area would in practice be largely at the discretion of the police, as they could close them down one-by-one if they chose to by mounting intensive breath testing campaigns.

Another interesting issue is what the police would do with offenders they catch if roadside breath testing equipment becomes sufficient to provide evidence in court. Currently they have to take suspects to the police station to test them on the Intoximeter, and they have to make their own way home later from there. However, if all they need to do is test them at the roadside, what do they do if they get a positive result? Clearly they can't then allow the offender to drive home, and presumably they would confiscate their keys. This would apply even if the offender had a BAC between 50 and 80, and was not going to receive a ban, because as soon as he started to drive again he would be committing another offence. It's not like a speeding offence when you can continue at a more circumspect pace. The police could attract some adverse publicity if they leave lone women stranded miles from anywhere and unable even to drive their own car.

It would be essential to produce statistics which distinguish drivers convicted of alcohol offences between those over 80 mg, and those between 50 mg and 80 mg. Only by doing this would it be possible to tell what effect the lower limit has had. If all alcohol offences were lumped together, it would suggest that there had been an initial upsurge which would be highly misleading and could even lead to pressure for yet harsher measures.


All things considered, I cannot see that a 50 mg limit would make any significant difference to drink-related casualties, because the vast majority of these casualties are caused by drivers who are well over the 80 mg limit and are (or should be) well aware of that fact, couldn't care less about laws and limits, and are extremely unlikely to modify their behaviour because they are not acting rationally in the first place. The only people who would change their behaviour are those who even now pose little or no additional risk on the roads. If road casualties did fall, then obviously I would be as pleased as anyone, but I would argue that the same reduction could have been achieved through means such as publicising the dangers of morning after driving and targeting high risk offenders, without any need to reduce the limit from 80 mg.

It could even make it worse, because if you replace a limit that is generally accepted with one that is not (particularly amongst potential offenders) then it is likely to lead to a greater degree of toleration for drink-driving per se. It could also lead to a reduction in the level of co-operation with the police. Currently, for example, most licensees of pubs with car-borne customers are willing to display anti drink-driving publicity material. Would they do the same after a limit reduction, particularly if they knew that a lot of their regular customers were driving when they were probably over 50 mg? There might also be an upsurge in "guerrilla resistance" to the new law in rural areas, in a way that at present has largely died out, making use of mobile phones and the Internet to track police stakeouts and enforcement campaigns.

(Last revised January 2005)

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