Traditionally, an approach known as the “four-step process” has been used for regional transportation planning analysis. As its name implies, this process has four basic phases: 1. Trip generation (the number of trips to be made); 2. Trip distribution (where those trips go); 3. Mode choice (how the trips will be divided among the available modes of travel); and 4. Trip assignment (predicting the route trips will take). By looking at these four areas, we can answer the following questions: How many trips will people make? Looking at land use characteristics (i.e. how we use land in our region) is the main way to figure out trip generation rates. This is because factors like the number and size of households, automobile ownership, types of activities (residential, commercial industrial, etc.), and density of development all drive how much travel flows from or to a specific area within the region. For simplicity, a geographic unit called a transportation analysis zone (TAZ) is used to create trip generation rates for the region. 

Mode choice shows which mode people use to travel between their origins and destinations. That is, whether people take transit, their own car, or a carpool to and from work or another destination. A complex sub-model has been created for determining the modal choice, and is based both on certain assumptions about transit capacity, schedules, and fare levels and on real-world observations of how, when, and where people use transit. What routes will people take? Trip assignment determines the routes people will take from start (origin) to finish (destination). Generally, the computer assumes everyone will take the quickest route to their destination. To compute route selection requires all kinds of information regarding actual or predicted congestion levels, road conditions, transit schedules and fares, traffic signal systems, etc.