escalator in European Union. The Prague metro is generally considered very safe. Roads The main flow of traffic leads through the centre of the city and through inner and outer ring roads (partially in operation).Inner Ring Road (The City Ring "MO"): surrounds central Prague. It is the longest city tunnel in Europe with a length of and five interchanges has been completed to relieve congestion in the north-western part of Prague. Called Blanka tunnel complex and part of the City Ring Road, it was estimated to eventually cost (after several increases) CZK 43 billion. Construction started in 2007 and, after repeated delays, the tunnel officially opened in September 2015. This tunnel complex completes a major part of the inner ring road.Outer Ring Road (The Prague Ring "D0"):''' this ring road will connect all major motorways and speedways that meet each other in Prague region and provide faster transit without a necessity to drive through the city. So far , out of a total planned , is in operation. Most recently, the southern part of this road (with a length of more than ) was opened on 22 September 2010. As of 2021, the next section between Modletice and Běchovice is planned to be completed in 2025. Rail The city forms the hub of the Czech railway system, with services to all parts of the country and abroad. The railway system links Prague with major European cities (which can be reached without transfers), including Berlin, Munich, Hamburg, Nuremberg and Dresden (Germany); Vienna, Graz and Linz (Austria); Warsaw, Wrocław and Cracow (Poland); Bratislava and Košice (Slovakia); Budapest (Hungary); Zürich (Switzerland); Split and Rijeka (Croatia, seasonal); Belgrade (Serbia, seasonal) and Moscow (Russia). Travel times range between 2 hours to Dresden and 28 hours to Moscow. Prague's main international railway station is Hlavní nádraží, rail services are also available from other main stations: Masarykovo nádraží, Holešovice and Smíchov, in addition to suburban stations. Commuter rail services operate under the name Esko Praha, which is part of PID (Prague Integrated Transport). Air Prague is served by Václav Havel Airport Prague, the largest airport in the Czech Republic and one of the largest and busiest airports in central and eastern Europe. The airport is the hub of carriers Smartwings and Czech Airlines operating throughout Europe. Other airports in Prague include the city's original airport in the north-eastern district of Kbely, which is serviced by the Czech