ticket system in the entire Stockholm County, which allows for easy travel between different modes of transport. The tickets are of two main types, single ticket and travel cards, both allowing for unlimited travel with SL in the entire Stockholm County for the duration of the ticket validity. On 1 April 2007, a zone system (A, B, C) and price system was introduced. Single tickets were available in forms of cash ticket, individual unit pre-paid tickets, pre-paid ticket slips of 8, SMS-ticket and machine ticket. Cash tickets bought at the point of travel were the most expensive and pre-paid tickets slips of 8 are the cheapest. A single ticket costs SEK 32 with the card and SEK 45 without and is valid for 75 minutes. The duration of the travel card validity depended on the exact type; they were available from 24 hours up to a year. As of 2018, a 30-day card costs SEK 860. Tickets of all these types were available with reduced prices for students and persons under 20 and over 65 years of age. On 9 January 2017, the zone system was removed, and the cost of the tickets was increased. The City Line Project With an estimated cost of SEK 16.8 billion (January 2007 price level), which equals 2.44 billion US dollars, the City Line, an environmentally certified project, comprises a -long commuter train tunnel (in rock and water) beneath Stockholm, with two new stations (Stockholm City and Stockholm Odenplan), and a -long railway bridge at Årsta. The City Line was built by the Swedish Transport Administration in co-operation with the City of Stockholm, Stockholm County Council, and Stockholm Transport, SL. As Stockholm Central Station is overloaded, the purpose of this project was to double the city's track capacity and improve service efficiency. Operations began in July 2017. Between Riddarholmen and Söder Mälarstrand, the City Line runs through a submerged concrete tunnel. As a green project, the City Line includes the purification of waste water; noise reduction through sound-attenuating tracks; the use of synthetic diesel, which provides users with clean air; and the recycling of excavated rocks. Roads Stockholm is at the junction of the European routes E4, E18 and E20. A half-completed motorway ring road exists on the south, west and north sides of the City Centre. The northern section of the ring road, Norra Länken, opened for traffic in 2015 while the final