downhill grade; downgrade
Refers to a downward slope, often used in contexts like roads, railways, or hiking trails.
この道は下り勾配が続くので、自転車で走ると気持ちいい。
This road continues on a downhill grade, so it feels great to ride a bicycle on it.
下り勾配ではスピードが出やすいから注意してください。
Please be careful because it's easy to pick up speed on a downhill slope.