China plans to relax minimum population requirements for cities that wish to build subway systems, opening up opportunities for a fresh bonanza of infrastructure investment, the state-owned Economic Information Daily reported on Monday. Citing unidentified authorities, the newspaper said that cities with an urban population of more than 1.5 million people would be allowed to build metro projects. Previously, only cities with a population of more than 3 million people and annual revenues of 10 billion yuan ($1.53 billion) were eligible. The relaxation underscores China's continued reliance on investment-led growth to support the slowing economy, which is credited by analysts as being behind recent signs of improvement in activity but also as creating a risk as debt levels rise. The newspaper said China's economic development had reached a new stage in which the fast expansion of cities was spurring traffic demand, and that population size should no longer be a key determinant of urban rail transit construction. Last week, China's transport ministry said the country will invest around 4.7 trillion yuan in transport infrastructure projects over the next three years. So far, 26 Chinese cities have built metro systems while another 39 have projects approved, the National Development and Reform Commission said in January.