A **speed bump** is a sudden change in the longitudinal profile of the road with the **concavity facing downwards**. This means that the road forms a kind of inverted "U", creating a depression in the path. Speed bumps are common in areas where it is necessary to reduce vehicle speed, such as in residential areas or near pedestrian crossings.

On the other hand, a **crest** refers to the slope of the road. A change of crest implies that the road changes slope, but does not necessarily form a concavity facing downwards. Changes of crest can be dangerous if visibility is reduced, as oncoming vehicles cannot be seen.

Finally, a **hump** is an elevation in the road, the opposite of a speed bump. Humps are used to force drivers to reduce speed, but they do not have the downward concavity characteristic of a speed bump.

Therefore, the correct term to describe a sudden change in the longitudinal profile with the concavity facing downwards is **speed bump**.