What impairment means for driver performance

Impaired driving is driving while alcohol, drugs or another substance has reduced a person's ability to perform the driving task. The research on alcohol is the most extensive. It shows effects on the skills driving depends on: dividing attention between tasks, searching the road scene, processing information, reacting to hazards, and controlling speed and lane position.

Those effects generally grow as the alcohol concentration rises, and tasks that require divided attention tend to be affected at lower concentrations than simple, well-practiced ones. Driving is a divided-attention task. A driver has to steer, hold a speed, watch other traffic and read the road at the same time, and an impaired driver may keep up one of those at the expense of the others.

How it bears on driving

Research has found that alcohol-impaired drivers tend to search the road scene less widely, concentrating on the area straight ahead and attending less to what lies to the sides. Hazards that appear away from the center of view, or that call for a choice between responses, are more likely to be detected late.

The presence of alcohol does not by itself show that impairment caused a crash. The analysis compares what the driver did with what a sober, attentive driver could have done in the same position: whether the hazard could have been seen and recognized sooner, and whether there was time to avoid it. Depending on the facts, that comparison can point in either direction. The same questions arise when the impaired person was a pedestrian or another road user.

Questions a human factors expert witness on impaired driving can address

  • Is the driver's conduct before the crash consistent with the known effects of impairment on attention and response?
  • Would a sober, attentive driver in the same position have detected the hazard and avoided the crash?
  • What did the driving task demand at that moment, and how would impairment be expected to affect it?

If a case raises questions like these, the answers depend on its particular facts and evidence. The practice is based in Bryan, Texas, and takes cases in Texas and nationwide. Counsel can begin with a case inquiry.

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This article is general information, not legal advice or an expert opinion on any matter. See the disclaimer.