Blood alcohol concentration (BAC) is a measure of how much alcohol is in your bloodstream. This measurement is used to determine whether a person is over the legal limit, which can be determined by a chemical breath or blood test. While .08% is the legal limit, people begin to feel the effects of alcohol well before that.
At this state, judgment is impaired, loss of muscle control, and decreased alertness will also be present. These effects obviously have an impact on one’s driving ability, including steering. The driver will also be less likely to respond appropriately to an unexpected emergency situation. Coordination is affected, as is visual tracking. Problems with visual tracking of objects is an indicator of inebriation, which is why horizontal gaze nystagmus testing is used during sobriety testing.
Muscle coordination diminishes even further, as does reaction time, speech, hearing, and reasoning. A person’s ability to process information is negatively impacted, while perception is also impaired. This may affect distance judgment, as the driver may believe a vehicle or other object in the road is further away than it appears.
Reaction time is deteriorated. Slurring is common at this stage, and a person may experience slowed-down thinking. Many people driving with a .10% BAC have difficulty staying within lanes, which makes their inebriation apparent to law enforcement.
Virtually all driving abilities are affected when a person has a BAC of .15%. Vomiting is likely, muscle control is well below normal, and balance is seriously compromised. Information processing, both visual and auditory, is also diminished.