This is the 6279th most frequent Russian word.
уменьшают
"Reduce" or "decrease."
In this context, 'уменьшают' refers to the gradual decrease in fatigue as a result of consistent physical activities.
Регулярные тренировки уменьшают усталость после работы.
Regular exercises reduce fatigue after work.
In this sentence, 'уменьшают' indicates the act of reducing the speed intentionally as an action taken by people.
Люди уменьшают скорость перед пешеходным переходом.
People reduce speed before a pedestrian crossing.
Here, 'уменьшают' describes how external natural factors cause a reduction in visibility.