La parte de la vía destinada a la circulación de coches es...

The part of the road intended for cars to drive on is the carriageway, according to the General Traffic Regulations. The carriageway is the part of the road used by vehicles in general, including cars, lorries and buses. The hard shoulder, on the other hand, is the side area intended for emergencies or temporary stopping, and the central reservation is the space that separates the two opposite directions of traffic. So the carriageway is correct, because that is where normal vehicle traffic takes place.