Although significant vehicle customisation was required, Fraport opted for the sophisticated Citaro series concept. Well over 30,000 units sold since its introduction in 1997 make this Mercedes-Benz regular-service bus the most successful ever. This success is due to a logical modular design concept which appeals to operators of regular-service buses. The modular design of the Citaro permits a high degree of flexibility in the manufacturing process and consequently the realisation of different vehicle lengths, superstructures, power levels, as well as numerous special versions and special design options, for example, door arrangements or door variants. The Citaro concept places special emphasis on ease of maintenance and the interchangeability of parts between the different models. These are factors which reflect positively in the life-cycle costs (LCC) over the long "life" of a bus, and they are also arguments that make the Citaro attractive as an airside transfer bus.. Furthermore, with its vehicle width of 2.55 metres, the Citaro is ideal for operating in the sometimes confined spaces at Frankfurt Airport. The extensive service network and the rapid response this can provide when help is required were also important factors in the purchase decision. .