Project Summary
In 2008, California passed the Sustainable Communities and Climate Protection Act of
2008 (Sustainable Communities Act, or SB375) that set regional targets for greenhouse
gas (GHG) emission reductions from passenger vehicle use. California’s Metropolitan
Planning Organizations (MPOs) are currently in the process of developing Sustainable
Communities Strategies (SCS), as part of their Regional Transportation Plans (RTP),
outlining strategies to meet greenhouse gas reduction targets adopted by the Air
Resources Board (ARB) in 2010.
MPOs use regional travel forecasting models to estimate vehicle trips (VT), average
speeds, and vehicle miles traveled (VMT), which serve in turn as input to regional
emissions models to estimate the GHG emissions. The state of the practice in regional
modeling, however, may limit the potential effectiveness of forecasting models to
accurately estimate interregional travel and the associated GHG impacts. There is but
limited empirical data on the proportion of interregional travel (measured by vehicle trips
and/or vehicle miles traveled), in part due to the definition of regional boundaries and
consistencies in external traffic counts at these boundaries. A rule of thumb is building
regional travel forecasting models is to have no more than 15 percent of total travel
crossing the regional cordon line. Even with this rule, proper estimation of interregional
travel may have a significant impact in overall estimates of GHG emissions.
A study is proposed to assess the state-of-the-practice in forecasting interregional travel
in California, to consider alternate methods to better represent interregional travel, and
to make recommendations on data needs and modeling policy.