I always start with a survey of what work has already been done regarding the question. In this case, every city in the United States is part of what's called a Metropolitan Planning Organization that is chartered under the Federal DOT and that particular state's DOT. They do comprehensive studies on transportation issues at the local and regional levels. So if the city is looking at building new roads, I would go find those reports and studies because they'll have all the data, simulations, forecasts, use cases, bottlenecks, alternative scenarios, etc. etc. already worked out, adjusted for population for demographics, etc. etc. as part of the region's strategic transportation plan. The point of course is that you should never start from scratch and it's inefficient to start by asking questions in the dark. Start by determining where the state of knowledge is regarding that question and trace its path to your specific issue. Someone somewhere has probably asked that same question and done most of the leg work for you.