Emissions Analysis Techniques for TCMs
Overview - The TCM Analyst is a spreadsheet-based sketch-planning tool developed by the Texas Transportation Institute (TTI) to estimate the emissions benefits of TCMs. It is based on methodologies previously developed for the U.S. Environmental Protection Agency ("EPA TCM Methodology").
Strategies Addressed - Improved transit; HOV lanes; carpooling and vanpooling promotion; telecommute and work hour strategies; traffic flow improvements.
Methodology - All calculations are performed in a Microsoft Excel workbook. Elasticities are used to relate changes in cost or time to travel changes, and to relate VMT changes to speed changes. The calculations account for trip lengths, prior mode of travel, etc. MOBILE emission factors by component (hot start, cold start, running, etc.) are applied to the outputs of the travel calculations. The methods typically use regionwide estimates of existing travel characteristics and calculate regional-scale effects, but they can also be applied at the corridor, facility, or zonal level.
Data Requirements - Data required for the analysis area include travel data (e.g., total person and vehicle-trips, trip distances, speeds for peak and off-peak periods); data on existing TCM implementation (e.g., average carpool size); census data (e.g., number of workers, persons per household, vehicle ownership); potential number of users; and MOBILE emission factors by speed, type, and operating mode. Various assumptions regarding elasticities or participation rates are also required for some strategies.
Outputs - Changes in trips, VMT, average travel speeds, and emissions.
Level of Effort - The baseline regional-level data requirements are fairly extensive and not all inputs will be readily available. Emission factors from over 400 MOBILE scenarios must be copied into the spreadsheet. The user may need to make other assumptions regarding participation levels, elasticities, induced trip-making, etc.
Advantages - By automating calculations, the spreadsheet format greatly simplifies the use of the EPA's methodologies. The methodologies provide a detailed accounting of impacts by emission component, vehicle type, work versus non-work trips, and peak versus off-peak travel. They also include factors to account for prior mode of travel, substitution effects (i.e., increased non-work driving by telecommuters), induced demand, and changes in trip lengths.
Limitations - Baseline regional travel data and emission factors can be somewhat cumbersome to develop. For some strategies, participation rates must be assumed by the user. Some effort may also be required to identify appropriate elasticities or other assumptions.
Source/Availability - The TCM Analyst 1.0 User's Guide and Software are available from the Texas Transportation Institute (979-845-4853, http://tti.tamu.edu/).
Reference: TCM Analyst 1.0 User's Guide. Research Report 1279-7, Texas Transportation Institute, College Station, TX (November 1994).
The following documents describe the underlying methodologies developed for EPA and provide examples of their application:
ICF Incorporated. Benefits Estimates for Selected TCM Programs. Prepared for U.S. EPA, publication no. EPA420-R-98-002, March 1999. Provides examples of the methodologies applied to six operating TCM programs.
Austin, et al. Methodologies for Estimating Emission and Travel Activity Effects of TCMs. Prepared by Systems Applications International for U.S. EPA, publication no. EPA420-R-94-002, July 1994. Available from the National Technical Information Service (NTIS) at 800-553-6847. Reference NTIS No. PB92-172566/REB.