Dhaka suffers some of the worst traffic congestion in the world. The city lacks an organized public transport system. Cycle rickshaws and auto rickshaws are the main mode of transport within the metro area, with close to 400,000 rickshaws running each day: the highest number in any city in the world. However, only about 85,000 rickshaws are licensed by the city government. Relatively low-cost and non-polluting cycle rickshaws are superior to private cars, which are exclusively responsible for Dhaka's congestion. The government has overseen the replacement of two-stroke engine auto rickshaws with "green auto-rickshaws", which run on compressed natural gas.