A naviduct is a special class of navigable aqueduct, in which the waterway also includes a lock. One example of a naviduct has been built at Enkhuizen on the Houtribdijk in the Netherlands on the instructions of the Rijkswaterstaat. This cost over €55 million and was completed in 2003. It is big enough to simultaneously transfer two large Rhine river-barges from the Markermeer to the IJsselmeer or vice versa. The structure was chosen because a busy waterway crosses a dam carrying a busy road, but the space available for separate aqueduct and lock facilities was restricted.