Final answer:
When a motorist has reached a BAC of .10%, their risk of causing a motor vehicle accident is significantly increased due to impaired coordination, judgement, reaction time, and decision-making abilities.
Step-by-step explanation:
When a motorist has reached a Blood Alcohol Concentration (BAC) of .10%, the risk of causing a motor vehicle accident is significantly increased. At this level of intoxication, the motorist's coordination, judgement, reaction time, and ability to make safe decisions are impaired, leading to a higher likelihood of accidents. It is important for motorists to understand that even a small amount of alcohol can impair their driving abilities, and it is always safest to avoid drinking and driving altogether.