Many viaducts over land connect points of similar height in a landscape, usually by bridging a river valley or other eroded opening in an otherwise flat area. Often such valleys had roads descending either side (with a small bridge over the river, where necessary) that become inadequate for the traffic load, necessitating a viaduct for "through" traffic.[4] Such bridges also lend themselves for use by rail traffic, which requires straighter and flatter routes.[5] Some viaducts have more than one deck, such that one deck has vehicular traffic and another deck carries rail traffic.