Background: Recent studies have revealed that brief but rapid accelerations can contribute significantly to a vehicle's carbon monoxide and hydrocarbon emissions during a typical urban trip. Current emission-factor models are insensitive to the distribution of such modal events (i.e., cruise, acceleration, deceleration, and idle) in the operation of a vehicle and instead estimate emissions by average trip speed.
The Clean Air Act Amendments (CAAA) of 1990 and the Intermodal Surface Transportation Efficiency Act (ISTEA) of 1991 place great emphasis on modeling to provide accurate accounting of progress toward meeting air quality goals and deadlines that, if not met, could lead to highway funds being withheld. Congestion mitigation and transportation management strategies will only be possible if it can be shown that their implementation will not further impair air quality in specific urban areas.
The current models, MOBILE and EMFAC, are being used to develop and evaluate transportation policy throughout the country under the authority of the CAAA. However, these models offer little help for evaluating operational improvements that smooth traffic flow such as ramp metering, signal coordination, and many IVHS strategies. Such operational improvements help reduce acceleration events and the queuing of vehicles, but they cannot be evaluated accurately by MOBILE or EMFAC because these models predict emissions only for average trip speeds.
Although some work by the U.S. Environmental Protection Agency (EPA) and the California Air Resources Board (CARB) is underway on modal-emission models, the most optimistic estimate for the implementation of a fully operational model is 5 years. Until an accurate and defensible modal-emissions model is available to demonstrate the air quality benefits of congestion relief, the fate of many operational programs and improvements will be determined by ill-suited models.
Objective: The overall objective of this research is to develop and verify a modal-emissions model that accurately reflects impacts of speed, engine load, and start conditions on emissions under a comprehensive variety of driving characteristics and vehicle technologies. The research will be divided into three phases. The objectives of Phase I are as follows: (1) conceptualize a modal-emissions model, (2) develop an interim working model, and (3) define the research plan needed to develop the model into an effective tool. This tool should be able to reliably estimate emission impacts from changes in driving characteristics associated with traffic operations and transportation system improvements. The objectives of Phase II are to collect data and to refine, test, and validate the modal-emissions model. The Phase III objective is to demonstrate that the model is responsive to the regulatory compliance needs of transportation and air quality agencies.
Tasks: Phase I: (1) Investigate and summarize existing literature concerning factors in the vehicle operating environment that may affect modal emissions, such as, hydrocarbon (HC), carbon monoxide (CO), and nitrogen oxides (NOx). At a minimum, factors addressed will include vehicle technology, fuel formulation (for example, reformulated gasoline), operating mode (cold starts, hot starts, acceleration), vehicle maintenance (inspection and maintenance programs), accessories (air conditioning), and road geometry (such as, steep grades). (2) Use existing data to define the domain and distribution of the modal parameters of cruise speeds, acceleration rate, idle, and deceleration rate for all facility types (freeway, highway, main arterial). (3) Critically evaluate existing models to determine if any meet the project objective or whether a new model is needed. The evaluation should consider ongoing regulatory changes in emissions certification procedures and compatibility with conventional emissions-factor models. The evaluation should also include a review of related work being conducted by the EPA and the CARB. Based on this evaluation, select the modal-emissions model type most suitable for accomplishing the project objective. (4) Based on Tasks 1, 2, and 3, design a testing protocol that will measure vehicle modal emissions over the domain of cruise, acceleration, idle, and deceleration conditions. Prepare an interim report that summarizes Tasks 1 through 4. The interim report will be distributed to the project panel for review and comment. (5) Using the protocol developed in Task 4, conduct preliminary testing on a representative sample of vehicles now in use. These data shall supplement existing data for the purpose of developing an interim working model. (6) Develop an interim working model applicable for analysis in the years 1990 to 2020 that covers the full domain of vehicle activities and significant factors affecting modal emissions. Demonstrate potential capabilities, limitations and maintenance requirements of the model. (The contractor will provide the interim working model and an interim report on Tasks 5 and 6 for NCHRP panel review and approval. The contractor will be required to make a presentation to the project panel. The contractor will not proceed with Phase II until NCHRP has approved the interim report and working model.)
Phase II: (7) Adjust the testing protocol developed in Task 4 for use in this task. Conduct testing compatible with the model developed in Task 6 on a sample of currently used vehicles. The vehicle sample shall represent the national fleet, including factors such as vehicle maintenance, vehicle type, vehicle age, mileage accrual, technology group, and emitter category. The sample size should be adequate to develop a comprehensive national model. (8) Using the data set collected in Task 7 and any other appropriate data available, examine the model developed in Task 6 and modify it as necessary to produce a final version of the model. (9) Verify, with an independent data set representative of real on-road conditions, that the testing method used adequately represents on-road emissions. The contractor will prepare an interim report that documents the research performed in Phase II for review and approval by the NCHRP panel.
Phase III: (10) Demonstrate how the model can be used with travel forecasting and traffic models in the design and evaluation of transportation systems and operational improvements. Also demonstrate how the modal-emissions model might be used to replace the current average speed correction algorithms in MOBILE and EMFAC. (11) Submit a final report that documents the entire research effort. This shall include an instructor's manual and user's manual for the modal-emissions model and the model software.
Phase IV: (12) In order to evaluate potential future year vehicle fleets and establish several new vehicle/technology categories that will have emission impacts, recruit and test 35-40 newer category vehicles. (13) Carry out further refinements to the modal emissions model based on the new tests. (14) Develop and carry out beta testing of the modal emissions model. (Introduce the modal emissions model to a transportation/air quality practitioners workshop.)
Status: The project is complete.
Product Availability: The model, user information, and database are available on CD-ROM from the principal investigator. The revised draft final report was published as NCHRP Web-Only Document 122 and can be accessed at https://www.trb.org/news/blurb_detail.asp?id=9018. Appendixes B and C of the final report will be published as an NCHRP report. The Comprehensive Modal Emissions Modal (CMEM) is currently available on the agency's website at https://www.cert.ucr.edu/cmem . Enhancements and additions subsequent to project completion have been made.