Experts say they have discovered a way that drinking alcohol impairs driving ability.
Drivers should be able to judge distances
As well as blighting decision-making powers and balance, it disrupts one's ability to judge depth, the North Dakota State University team found.
Drivers who are under the influence of alcohol may not be able to gauge distances, leading to accidents, according to the researchers.
Their findings appear in the journal Psychological Science.
Dr Mark Nawrot and colleagues looked at a particular type of depth perception, called motion parallax.
This is information about the relative depth of objects gleaned from our own motion.
For example, under normal conditions, a driver should be able to judge how far away a tree or another vehicle is from their own moving car.
It is well known that binocular vision is important for depth perception.
To see objects clearly we move our eyes so that the fovea, the area with the best visual acuity, is positioned upon the area of interest.
These movements are produced by two different systems - the fast and slow eye movement systems.
Fast eye movements allow us to dart our eyes to where we want.
Slow eye movements allow us to fixate and track moving objects.
Alcohol impairs both eye systems.
The slow eye movements become too slow, and the body then uses the fast system to catch up, which produces jerky eye movements, called horizontal nystagmus.
The latest study shows motion parallax relies heavily on the slow eye movement system.
The researchers asked 15 volunteers to perform tasks that measured perception of depth from motion parallax before and after drinking alcohol.
LEGAL ALCOHOL LIMITS
35 microgrammes of alcohol in 100 millilitres of breath
80 milligrammes of alcohol in 100 millilitres of blood
107 milligrammes of alcohol in 100 millilitres of urine
There is no failsafe guide as to how to stay under the legal alcohol limit
The only safe option is not to drink if you plan to drive
Source: THINK! Road Safety
As expected, when the volunteers were intoxicated their ability to judge depth from motion parallax was impaired.
Dr Nawrot said: "The current study shows that an important perceptual system for driving is compromised by alcohol intoxication.
"This may be one part of a broader, but more poorly understood, set of visual perception problems caused by ethanol's effect on the eye movement system."
He said a quick decision based on faulty or inadequate perceptual information might be a critical component in many alcohol-related driving accidents.
In 2003, 560 people were killed in drink drive related crashes, 2,600 were seriously injured and 19,000 were slightly injured.