routes 507 and 521 in May 2009. The last were withdrawn on 9 December 2011. In May 2010, Mayor of London Boris Johnson unveiled the design of the New Routemaster, the proposed replacement for the Routemaster as an iconic standard bus for exclusive use in London. The buses, designed by Heatherwick Studio and built by Wrightbus feature two staircases, three doors and an open platform allowing passengers to hop on and off, and commenced operating in 2012. In December 2011 the British car magazine Autocar praised the New Routemaster in a road test, rating it ahead of contemporary and historic buses. However, in December 2016 the new Mayor of London Sadiq Khan decided that no more orders would be placed for the bus after only 1,000 of Johnson's envisaged fleet of 2,000 had been procured. In 2020 Transport for London announced that the New Routemasters would be converted so passengers only enter by the front door, with the middle and rear doors becoming exit-only. This was done to reduce fare evasion, which had been double that of other London buses. Transition to zero emission bus fleet Since the early 1990s, efforts have been underway to reduce the emissions of the bus fleet. Early work involved replacing older buses like the AEC Routemaster and fitting particulate filters to exhausts. In the 2000s, hybrid and hydrogen fuel cell buses were trialed - and the first hybrid buses entered service in 2006. It was originally intended that every bus introduced into service after 2012 would be a hybrid, but this requirement was later dropped. Battery electric buses first entered service in 2014, and double decker hydrogen fuel buses were introduced in 2021. London now has the 2nd largest bus fleet in Europe with over 800 battery electric and hydrogen fuel cell buses in service. In 2021, it was announced that all buses in the fleet meet or exceed Euro VI emission standards, following the phasing out of older buses, the retrofitting of diesel vehicles and the introduction of new hybrid & electric buses. The Mayor of London is currently aiming for a zero emission bus fleet by 2037. From the autumn of 2020, all new single deck buses entering the fleet are either electric or hydrogen zero emission buses. Operation Local buses Most local buses within London form a network managed by London Buses, an arm of Transport for London. Services are operated