Greenhouse Gas (GHG) and Climate Change

Climate Change can be felt through increasing severity and longer duration of major heat wave, drought, wildfire, high wind, heavy rainfall, and higher storm surge as a result of sea level rise. Potential impacts to the State Highway System include flood, washout, landslide, pavement deterioration, slope instability, bridge scour, and increased wildfires. As California faces these extreme weather events, the transportation infrastructures must be prepared to withstand such forces of nature in order to maintain the lifelines of the State. Efforts are currently underway to plan and implement a resilient transportation system to reduce the impacts of Climate Change. State and regional transportation agencies are incorporating resiliency needs into the transportation planning processes while minimizing transportation-related fuel consumption and air pollution.

The predominant driver of Climate Change is greenhouse gas (GHG). The transportation sector is a major contributor of greenhouse gas (GHG) emissions. Mobile source GHG includes carbon dioxide (CO2), methane (CH4), nitrous oxide (N2O), black carbon (BC), and fluorinated gases including hydrofluorocarbons (HFCs). While GHG regulations can differ in some aspects than the other air pollutants, mitigation measures to reduce GHG have been noted to also reduce air pollutants. Energy and fuel consumptions are major factors of a GHG analysis. The same fuel combustions that produce air pollutants also generate GHG. As such, many of the tools available to estimate GHG emissions are the same as air quality analysis.

On-Road GHG Emissions

The analysis approaches for on-road GHG emission differ depending on the context of the discussion. Caltrans prefers to conduct project analysis using a travel activity-based approach that relies on speed-distributed travel activity data and gram-per-mile (g/mi) emission factors from EMFAC. Regional analyses and the annual statewide GHG inventory prefer to use a fuel-based approach that relies on fuel sales data and the carbon content of the fuel mixtures. For project-level application, the fuel-based approach has limitations in developing project-specific fleet fuel economy and reflecting variations in project travel activities (e.g., speed changes during peak and off-peak time periods).
    The ARB EMFAC model includes CO2 and CH4 emissions. Typically, the EMFAC-PL module would be used to conduct a project-level analysis.
    Caltrans developed the CT-EMFAC to combine the emission factors from EMFAC with project-specific vehicle activity data (e.g. traffic volume, speed, and fleet mix) to calculate emissions for multiple project scenarios (e.g. no-build and build alternatives), roadway links, and time period.  
    CT-EMFAC is not available on the internet but available by request from

Construction GHG Emissions

GHG analysis may entail a full life-cycle assessment of a project, from the initial construction to long-term operation and maintenance of the facility. Construction GHG emissions may include material processing and delivery, on-site construction equipment, and traffic delays due to construction. To estimate construction GHG emissions, the following tools are available for consideration.
  • Caltrans Construction Emissions Tool (CAL-CET)
    Caltrans developed the CAL-CET to analyze construction emissions for various types of highway improvement projects that Caltrans administers annually statewide. The methodology and assumptions are based on Caltrans-specific data and construction practices, knowledge, and experiences. For non-Caltrans projects, consideration should be given to account for differences between Caltrans equipment rental rates and construction fleets and the lead agency’s construction practices.
    CAL-CET is not available on the internet but available by request from
  • Road Construction Emission Model (RCEM)
    The RCEM was developed by the Sacramento Metropolitan Air Quality Management District (Sac Metro) as a part of their CEQA Guidance & Tools to analyze construction emissions for roadway projects within the Sacramento region. For projects outside of the Sac Metro jurisdiction, please be aware that RCEM utilizes emission factors specific to the Sacramento region.
  • FHWA Infrastructure Carbon Estimator (ICE)
    The ICE was developed by the FHWA to estimate the lifecycle energy and GHG emission from construction and maintenance of transportation facilities. It is based on national average data.
  • California Emissions Estimator Model (CalEEMod)
    The CalEEMod was developed for the California Air Pollution Officers Association in collaboration with several local air districts. This is a statewide land-use emission model that include a construction emission module. CalEEMod's primary focus is on land development projects more so than roadway projects. It is more viable for projects that involve building structures or parking lots facilities, such as offices, safety roadside rest areas, park and rides, or transit hubs. 

Additional References