Aqueducts were lengthy conduits built by the Romans to transport water into towns. Many Roman aqueducts were underground. The water brought into towns was utilized for drinking water, bathing, and sewage. The majority of aqueducts were made out of stone but some were made of wood or metal. The breadth of Rome's roads increased every year from about 250 BC until they were finally declared illegal in 194 AD. By this time they spanned more than 2000 miles. They were used for military purposes as well as travel by pedestrians and vehicles. Rome's many bridges were also important for transportation. There were five types of bridges that were built over various waterways within the city limits: arches, semi-subterranean, suspended, half-overhead, and double bridge. Arches were used where there was not enough space for a large opening. They resembled a small doorway with supporting posts beneath it. Arches could be used for footbridges, street bridges, and railway bridges. Semi-subterranean bridges were built below ground level but with open-air corridors on both sides. They could be accessed by stairs or ramps. Semi-subterranean bridges were commonly used for highway bridges.