fires resulted in their withdrawal from service, and they have also been responsible for causing an increase in traffic congestion and accidents involving pedestrians and cyclists. United Kingdom Until 1980, articulated buses were illegal on the UK's roads. A 1979 experiment by South Yorkshire Passenger Transport Executive with buses manufactured by MAN and Leyland-DAB led to a change in the law, but the experiment was abandoned in 1981 because double-decker buses were generally considered less expensive both to purchase and to operate. The cost and weight of the strengthened deck framing and staircase of a double-decker was lower than the cost and weight of the additional axle(s) and coupling mechanism of an articulated bus. Modern technology has reduced the weight disadvantage, and the benefits of a continuous low floor allowing easier access plus additional entrance doors for smoother loading have led to reconsideration of the use of articulated buses in the UK. In London, articulated buses were used on some routes from 2001 until 2011, but they were not a success. Boris Johnson, former Mayor of London, promised in the run-up to the mayoral election of 2008 to rid the city of the controversial buses and replaced them with double deckers. Elsewhere in the UK, they are generally operated on particular routes in order to increase passenger numbers, rather than across entire networks. With unsupervised "open boarding" through three doors and the requirement for pre-purchase of tickets, levels of fare-dodging on the new vehicles were found to be at least three times higher than on conventional buses where entry of passengers is monitored by the driver or conductor. The only way of checking for free riders was to use large teams of ticket inspectors to swamp the bus and inspect all tickets while keeping the doors closed, meanwhile delaying the further progress of the bus. Since the articulated buses were tending to serve areas of relative deprivation it is suspected that this was a contributory factor in Transport for London (TfL) turning against the concept. Many of the articulated buses from London went on to serve with regional operators. Aside from limited use in regional cities, articulated buses may now be found at airports as park and ride shuttles. In 2020, twenty one brand new Mercedes Benz Citaros entered service at Stansted Airport; the Mercedes Benz Citaro is probably the only articulated bus available in the United Kingdom at present.