Buses are the most popular mode, carrying 872 million passengers. The network is extensive, made up of 755 km within the city proper, and 70 km within the greater Budapest region. There are many bus-only lanes, and one quarter of the routes have rush- hour limited stop service for efficiency. Articulated buses are also used on busy routes to accomodate passengers. Buses are the most suitable mode of transport for hilly Buda, and other areas not easily accessible, such as areas which have been developed recently, and not yet connected with Metro, trolley, or tram service. While buses are the most common type of transportation, their popularity is declining. As traffic congestion keeps increasing in Budapest's outdated road system, they are frequently prone to delays. Buses are also losing support because they are the most environmentally unfriendly transit option in a city which is becoming more conscientious of its massive pollution problem, of which buses are a major source.