What is a common effect of driving while drowsy?

Driving while drowsy is known to significantly impair a driver's ability to react quickly and effectively to situations on the road. Slowed response time occurs as fatigue affects cognitive functions, making it harder for a driver to process information and respond to changing driving conditions. This lethargy increases the likelihood of accidents, since a drowsy driver may be unable to react promptly to obstacles, traffic signals, or other vehicles.

In contrast, the other options suggest improvements or enhanced abilities, which is not aligned with the impacts of drowsiness. Rather than improving concentration or increasing attentiveness, drowsiness typically leads to a decline in performance and awareness. Thus, recognizing the effect of slowed response time is critical to understanding the dangers of driving while tired.