Oversized/ Overweight Heavy-Duty Vehicle Emission Impact Study

The Dallas-Fort Worth (DFW) region is in violation of federal ambient air quality standards for ozone. The reduction of oxides of nitrogen (NO_x) emissions, an ozone precursor, is an important part of achieving regional air quality goals. Heavy-duty vehicles (HDVs) are a major contributor of NO_x emissions, contributing a significant proportion of total on-road emissions. Within the HDV population, it is believed that oversize (OS)/overweight (OW) vehicles generate higher levels of emissions than an average HDV due to excessive load on the engine during OS/OW operations. However, there is limited existing information/knowledge on this subject. Therefore, it is important to develop a systematic approach to understanding the contribution of these vehicles to regional emissions and to develop appropriate policies to reduce OS/OW NO_x emissions.
This is an image of a 18-wheeler truck at Rellis Texas A&M University.

The overall goal of the project was to estimate the emissions impacts of OS/OW vehicles operating in the DFW non-attainment (NA) area. The project objective was accomplished through the completion of three main activities:

Characterization of OS/OW operations in the DFW region—To determine the impact of OS/OW operations, the research team first looked at the status of OS/OW operations in the area. This task included analyzing currently available data on OS/OW activities in the area, including weigh-in-motion (WIM) data from the Texas Department of Transportation (TxDOT), OS/OW permit data from the Texas Department of Motor Vehicles (TxDMV), and data on commercial vehicle weight-related violations from the Texas Department of Public Safety (TxDPS).
OS/OW activity and emissions data collection—To determine the impact of OS/OW activities, the research team collected and analyzed vehicle activity and emissions data from a sample of OS/OW vehicles. The data collection effort included real-world activity data collection and emissions testing using portable emissions measurement systems (PEMS).
Analysis and estimation of OS/OW vehicle operations and emission rates—The collected field data were used to develop emission rates for different OS/OW scenarios used in the analysis to quantify the emissions impacts of OS/OW operations in the DFW area. The analyses were based on methods used in the U.S. Environmental Protection Agency’s (EPA’s) MOtor Vehicle Emissions Simulator (MOVES) model.

Oversized/ Overweight Study Final Report [PDF]
Oversized/ Overweight Study Final Report Appendices [PDF]

Appendix A includes a brief history of OS/OW regulations, the current limits set for permitted and non-permitted loads, the type of permits issued in Texas, and the movement restrictions for operating permitted loads.

Appendix B includes information on OS/OW vehicle operations from sources identified during the literature review, including recent studies on OS/OW vehicle operations in Texas.

Appendix C provides the WIM data from the WIM stations in the NCTCOG area by vehicle classification.

Appendix D provides a summary of other studies that evaluated the emissions impacts of OW vehicles.

Appendix E describes the study design plan that guided the data collection efforts that were conducted for the study.

Appendix F describes the data-processing steps used to obtain the emission rate information from the raw emissions data collected during the testing task.

Appendix G covers the unauthorized OS/OW field campaign and results from the data collection.

Appendix H provides all the emission rates calculated from the data collected during the testing of a sample of OS/OW vehicles.