How can ABAG and MTC predict the future?

We do not predict the future. For several decades, both MTC and ABAG have been developing and updating long-term regional plans for the Bay Area by using computer modeling to forecast transportation and housing demand, economic growth, demographics, and land-use changes, among others.  These forecasts are used to inform planning and investment decisions. The forecasts are updated every two to four years to make sure they are based on the most reliable data, including locally adopted plans for development and conservation.