The Dom Luis I Bridge is a double-decked metal arch bridge that spans the Douro River between the cities of Porto and Vila Nova de Gaia in Portugal. At the time of construction its span of 172 m was the longest of its type in the world. The top deck has been occupied by Line D of the Metro do Porto light rail system (which opened in 2005) and a pedestrian walkway, offering great views on the historic center of Porto, the Port wine caves of Vila Nova de Gaia and the river Douro.