Public transportation consumes 3.7 times less energy per passenger than private modes of transportation, according to a study by the International Association of Public Transport (UITP). The study, “Millennium Cities Database of Sustainable Development,” looked at 100 cities worldwide. The most energy-efficient country is Japan, where public transportation is as much as 10 times more efficient than private modes of transportation. In such cities as Madrid and Barcelona, levels of energy use per passenger kilometer are 0.71 and 0.37 mega joules, respectively. That is compared with the 2.71 and 2.25 mega joules used for private transportation in those cities. UITP’s study also shows that cities with efficient public transportation systems are much less damaging to the environment. Out of the 35 cities studied in western Europe, the one with the highest carbon monoxide (CO) emissions is Bologna, with 206.10 kilograms per capita. At 21.62 kilograms, the city with the lowest emissions is Amsterdam, where the bicycle is a popular form of transport. Emissions levels in the United States, where the automobile is dominant, are consistently higher. Annual CO emissions in Atlanta and Houston are 399 and 243.70 kilograms per capita, respectively.