Transportation Models

Activity-Based Model

SCAG is the primary agency responsible for the development and maintenance of travel demand forecasting models for the SCAG region. SCAG has been developing and improving these travel demand forecasting models since 1967. SCAG applies the models to provide state of the practice quantitative analysis for the Connect SoCal Regional Transportation Plan/Sustainable Communities Strategy (RTP/SCS), the Federal Transportation Improvement Program, and air quality management plans. The regional model is also used to evaluate other transportation proposals within the region. The model is based on Caliper Corporation’s TransCAD modeling software and the latest generation of the Coordinated Travel – Regional Activity Modeling Platform. 

The SCAG travel demand process is composed of two main components: 

1. The SCAG Activity-Based Model (ABM), which simulates daily activity participation and scheduling for individuals, with travel viewed as a derivative of out-of-home activity participation and scheduling decisions. 

2. A network assignment loads vehicles onto appropriate facilities to produce traffic volumes, congested speeds, vehicle-miles traveled, and vehicle-hours traveled estimates for each of the five time periods. The SCAG travel demand modeling process is shown in Figure 1. A series of multi-class highway assignments simultaneously load the vehicle forecasted by the SCAG ABM, pre-calculated trip origin and destination input matrices (airport, seaport, inter-regional by passenger vehicles), and three classes of heavy-duty trucks from the Heavy Duty Truck model. 

Heavy-Duty Truck Model

SCAG developed the Heavy-Duty Truck Model to evaluate important policy choices and investment decisions. The Heavy-Duty Truck Model is a primary analysis tool to support the goods movement policy decisions made by SCAG and regional stakeholders. 

As part of the Connect SoCal 2024 Regional Transportation Plan/Sustainable Communities Strategy, the Heavy-Duty Truck Model focused on analytical needs to support project and policy planning.  

The model development process included a thorough review of the Heavy-Duty Truck Model by an expert panel that ranged from academics to consultants. The Heavy-Duty Truck Model was validated by comparing the overall vehicle miles traveled for individual counties and air basins against the data obtained from the Highway Performance Monitoring System and screenline count. 

The model documentation includes further information on the following topics: 

• Heavy-Duty Truck Model Structure: The model forecasts trips for three heavy-duty truck (HDT) weight classes: light-heavy (8,500 to 14,000 lbs. gross vehicle weight (GVW); medium-heavy (14,001 to 33,000 lbs. GVW); and heavy-heavy (>33,000 lbs. GVW). 
• Internal Heavy-Duty Truck Model: This includes all HDT trips that have both origins and destinations within the six-county modeling area. 
• Ports HDT Trips: This includes truck trips coming out of and going into the airports and seaports in the region. 
• External Heavy-Duty Truck Model: This includes the external HDT trips coming into, going out of, and passing through the region. 
• Heavy-Duty Truck Model Calibration and Validation: This includes validation data, procedures, and results from various performance measures. 
• Other components include time-of-day factoring, multi-truck classification assignment, and HDT composite operating cost.  

Air Quality Modeling & Analysis

SCAG is responsible for the Transportation Air Quality Conformity Determination for the Connect SoCal 2024 Regional Transportation Plan/Sustainable Communities Strategy and the Federal Transportation Improvement Program. These conformity responsibilities now include the new air quality standards for fine particulate matter (PM2.5) and 8-hour Ozone. 

California’s adoption of Senate Bill (SB) 375, the Sustainable Communities and Climate Protection Act of 2008, assigns SCAG planning and modeling responsibilities. SB 375 implements the state’s greenhouse gas emissions (GHG) reduction goals in the sector of cars and light trucks. This mandate requires SCAG to meet its per-capita GHG emissions reduction targets at two points in the future: 2020 and 2035.