How does alcohol impair drivers?

Driving is a more complex task that many people realize. Often it seems a monotonous task punctuated by an unpredictable demand for decision and reaction. Common driving tasks like steering and braking are themselves complex events involving perception, information processing, reaction time, and eye-to-hand coordination.

Safe drivers are preoccupied with guiding the car properly through the lane of travel, but they must also scan the roadway and area ahead for cars, pedestrians, and any other hazards.
Normally we can perform any driving task easily and safely, but some driving tasks are more difficult than others. Driving down a lightly traveled road in daylight in good weather and with little cross-traffic can be an easy drive. But driving in downtown Vancouver on a rainy winter evening at rush hour in heavy pedestrian and car traffic is a very different experience.
Driving mistakes can happen especially when the driving conditions are challenging. They can also happen when we are distracted by other tasks, such as talking to a passenger or using a cell phone. Driving errors also happen when our capacity to process information is diminished by the presence of impairing substances in our body, such as alcohol.
The effect of alcohol on driving has been studied for decades, and it is well established that alcohol impairs our attention, comprehension, and judgment. These effects cause drivers to miss important information from the driving environment and to take greater risks while driving. Moreover, alcohol slows the rate of information processing in the brain and this can result in a delay in responding to an emergency situation while driving.
Impaired drivers are more likely to make errors when the driving task is difficult than when the driving is straightforward and easy. The most difficult driving task may occur when an emergency situation arises requiring the driver to respond very quickly and appropriately.
Lastly, it is important to remember that impaired drivers do not always crash. Moreover, sometimes an impaired driver who is involved in an accident did not cause the crash and is not at fault. Likewise, sober drivers with no alcohol and zero BACs can make driving errors that lead to a crash.
For most people, the impairing effects of alcohol at BACs of 30 mg% or less are barely measurable and the risk of a crash is low. But the crash risk increases rapidly as the BAC rises. At BACs of 50 mg% drivers are 1.4 times more likely to be involved in a crash as a sober driver; the risk increases to 2 times the sober driver at 70 mg%; while above 100 mg%, the risk increases dramatically and almost exponentially.