which costs differ. Criticism BRT systems have been widely promoted by non-governmental organizations such as the Shell-funded EMBARQ program, Rockefeller Foundation and Institute for Transportation and Development Policy (ITDP), whose consultant pool includes the former mayor of Bogota (Colombia), Enrique Peñalosa (former president of ITDP). Supported by contributions of bus-producing companies such as Volvo, the ITDP not only established a proposed "standard" for BRT system implementation, but developed intensive lobby activities around the world to convince local governments to select BRT systems over rail-based transportation models (subways, light trains, etc.). "Fake" BRT systems (BRT creep) Bus rapid transit creep is a phenomenon commonly defined as a bus rapid transit (BRT) system that fails to meet the requirements to be considered "true BRT". These systems are often marketed as a fully realized bus rapid transit system, but end up being described as more of an improvement to regular bus service by proponents of the "BRT creep" term. Notably, the Institute for Transportation and Development Policy (ITDP) published several guidelines in an attempt to define what constitutes the term of "true BRT", known as the BRT Standard, in an attempt to avert this phenomenon. The most extreme versions of BRT creep lead to systems that cannot even truly be recognized as "Bus Rapid Transit". For example, a rating from the ITDP determined that the Boston Silver Line was best classified as "Not BRT" after local decision makers gradually decided to do away with most BRT-specific features. The study also evaluates New York City's Select Bus Service (which is supposed to be BRT-standard) as "Not BRT". Environmental issues Unlike electric-powered trains commonly used in rapid transit and light rail systems, bus rapid transit often uses diesel- or gasoline-fueled engines. The typical bus diesel engine causes noticeable levels of air pollution, noise and vibration. It is noted however that BRT can still provide significant environmental benefits over private cars. In addition, BRT systems can replace an inefficient conventional bus network for more efficient, faster and less polluting BRT buses. For example, Bogotá previously used 2,700 conventional buses providing transportation to 1.6 million passengers daily, while in 2013 TransMilenio transported 1.9 million passengers using only 630 BRT buses, a fleet less than a quarter in size of the old fleet, that circulates at twice the speed, with a huge reduction in air pollution. To reduce direct emissions some systems use alternative forms of traction such