Ralentisseur vs. dos d'âne
While both terms refer to obstacles used to reduce vehicle speed, ralentisseur is the broad technical category for any traffic calming device, whereas dos dâne' describes the specific hump-shaped variety often found on streets.
Ralentisseur
B1This is the generic, formal, and administrative term derived from the verb ralentir (to slow down). It works as an umbrella term that covers all types of speed-reducing infrastructures, including cushions, raised tables, and humps.
La mairie a décidé de placer un ralentisseur devant l'école primaire.
(The town hall decided to place a speed breaker in front of the primary school.)
Il faut respecter la limitation de vitesse à l'approche du ralentisseur.
(You must respect the speed limit when approaching the traffic calming device.)
Ce type de ralentisseur est conçu pour ne pas gêner les bus.
(This type of speed bump is designed not to hinder buses.)
Les panneaux indiquent la présence de plusieurs ralentisseurs sur cette route.
(The signs indicate the presence of several speed breakers on this road.)
Dos d'âne
B2Literally translating to "donkey's back," this term refers to the classic short, rounded hump shape of a speed bump. It is the most common colloquial term used by drivers in everyday conversation to describe standard speed bumps.
J'ai failli abîmer ma voiture en passant trop vite sur ce dos d'âne.
(I almost damaged my car by going too fast over this speed bump.)
Ce dos d'âne est vraiment trop haut pour les voitures de sport.
(This speed bump is really too high for sports cars.)
Attention, il y a un gros dos d'âne juste après le virage.
(Careful, there is a big speed bump right after the turn.)
Les résidents réclament un dos d'âne pour empêcher les gens de rouler vite.
(The residents are demanding a speed bump to stop people from driving fast.)
Summary
Use ralentisseur when speaking technically or referring to the general concept of traffic calming. Use dos dâne' in daily conversation to specifically describe the physical hump in the road.