Measurements of Drowsiness and Blood Alcohol Concentration in Drivers

Tuesday, July 29, 2014 by Dr Murray Johns

Over the past few decades the number of road crashes per annum (and the number per 100,000 vehicles) has been greatly reduced in many countries. The reduction can be attributed to several reasons including the control of drunk driving. There has been widespread adoption of road-side breathalyser testing and of legislation making it illegal to drive with a blood alcohol concentration (BAC) above a critical level, typically 0.05 – 0.08 g/100 ml in different countries.

At the same time, it has been recognised that drowsy driving is also a significant cause of road crashes, being the major factor in about 20% of highway crashes. New methods, such as Optalert’s early-warning drowsiness detection technology, have recently been developed for measuring levels of alertness/drowsiness objectively and continuously in drivers. Experiments under controlled laboratory conditions have shown those measures of alertness/drowsiness are related statistically to the person’s BAC at the time. That is, at least some of the effects of blood alcohol on brain function are mediated by reduced alertness.

This has raised expectations among some people those measurements of alertness/drowsiness might be useful as the basis of a road-side test, comparable to the measurement of BAC by a breathalyser. It might then be possible, theoretically, to detect drivers who are too drowsy to drive at a particular time by measuring their drowsiness by a simple and brief road-side test. Unfortunately, things are more complicated than that. Current measures of drowsiness and of BAC are fundamentally different as risk factors in drivers.

Measures of BAC are not influenced significantly by the person’s posture, behaviour and cognitive state at the time, whereas currently available measures of drowsiness definitely are. Just think of the effect on a driver who has just had a few drinks of being apprehended by a policeman. That confrontation will not reduce the driver’s BAC significantly over the next few minutes. We can determine by a breathalyser test that the driver had a particular BAC while driving before he was apprehended. By contrast, confronting the policeman will almost certainly make the driver more alert, at least for a few minutes.

Currently we do not have an accurate measure of drowsiness short enough to be practically useful as a road-side test, and that is able to determine whether the driver was too drowsy to drive before he was apprehended. He may have been very drowsy then, and not be very drowsy a few minutes later. Drowsiness can fluctuate very rapidly, in a way that BAC does not. That is why it is necessary to monitor the levels of alertness/drowsiness of drivers continuously, at least under some circumstances, to control the risks of drowsy driving.

The current Optalert eagle INDUSTRIAL drowsiness detection unit, however, does display a history of alertness warnings for individual drivers, so in future; it may offer alternatives for law enforcement. Officers may be able to use this history to determine whether a driver has been alert when driving or has been receiving warning signals they are in danger of entering the dangerous state of drowsiness.

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