Driving on a road with heavy traffic **promotes the onset of fatigue** for several reasons:

- **Dense traffic**: When there is a lot of traffic, drivers must constantly pay attention to the vehicles around them, which increases stress and mental fatigue. The need to repeatedly brake and accelerate also contributes to physical exhaustion.

- **Constant concentration**: In heavy traffic situations, it is necessary to maintain a high level of concentration for extended periods. This can be exhausting and lead to fatigue more quickly than in free-flowing traffic conditions.

- **Prolonged driving time**: Dense traffic usually extends travel time, meaning drivers spend more time behind the wheel, thus increasing the chances of feeling fatigued.

In summary, driving on a road with heavy traffic requires more mental and physical effort, which increases the likelihood of experiencing fatigue.