Bangkok has seen rapid urbanization since its population reached two million in the 1960s. Since the 1980s, greater Bangkok's built-up areas have spilled beyond Bangkok's borders to neighboring provinces, initially to the north and south. Despite a general suburbanization trend, Bangkok remained centralized and the city core remained extremely dense until the early 2000s as heavy commuter traffic limited choices. The countryside between once independent towns and the capital became ever more filled in, with the advent and expansion of urban rail transit, as well as cheap credit enabling automobile adoption by the working class. The outward push of suburbanization has intensified as park and ride lots near train stations have sprung up.