Urban planners in a rapidly growing city have proposed building a new metro line to reduce traffic congestion. The city's roads are severely overcrowded during peak hours, with average journey times having doubled over the past decade. Studies show that cities with extensive metro systems have lower rates of car usage and better air quality. The proposed line would connect residential suburbs to the business district, covering the route used by 100,000 daily commuters. Investment in public transport infrastructure has successfully reduced congestion in comparable cities. Building the metro line will therefore alleviate the city's traffic problems. Which one of the following is an underlying assumption of the above argument?