Project at a Glance

Title: Development of the CALIMFAC California I/M Benefits Model.

Principal Investigator / Author(s): Sierra Research, Inc.

Contract Number: A6-173-64

Research Program Area: Economic Analysis, Emissions Monitoring & Control

Topic Areas: Mobile Sources & Fuels, Modeling


A computer simulation model called CALIMFAC ("California I/M Factor") has been developed for use in evaluating the effectiveness of the California biennial vehicle inspection, or "Smog Check", program. The model calculates baseline (no inspection program) exhaust emission factors for 1965 to 2004 model year gasoline-powered passenger cars, light-duty trucks and medium-duty vehicles, and predicts emission benefits for calendar years 1980 to 2020 for up to five different sequential I/M program designs. Program options that can be evaluated include inspection frequency, inspection test type, visual/functional check, emission standard stringency, repair cost limits, mechanic performance, model years included, and specific, vehicle exemptions. A total of over 200 different I/M program designs can be constructed from the available options, with an infinite combination of start dates and exemptions. Options are selected by the user from a series of menus that prompt for input. Although the model was initially written for execution on a minicomputer, a personal computer version has also been prepared.

Emission factors predicted by the model are somewhat higher than those predicted by the Air Resources Board's emission factor model, EMFAC7D. This is probably due to the model's treatment of malperforming vehicles. It is believed that previous analyses underestimated emissions from malperforming vehicles. The model estimates that the enhanced Smog Check program resulting from the implementation of SB 1997 will reduce exhaust hydrocarbon (HC) emissions by nearly 18%, carbon monoxide (CO) emissions by about 19% and oxides of nitrogen (NOx) emissions by about 12% in 1992, when the program enhancements are fully implemented. HC benefits are predicted to remain fairly constant through 2020 at between 17 and 18%. CO reductions from the SB 1997 program continue to increase, leveling off at approximately 27% by 2012. NOx emission benefits are projected to peak in the early 1990s at approximately 12%, and then to level off about 2012 at a little over 5 percent. The model also shows that the SB 1997 program changes (two-tier mechanic licensing requirements, increased cost limit for repairs and computerized emission test analyzers) will result in both near-term and long-term program improvements, as shown in Table I-l below.

Table I-l
Smog Check Program Benefits Projected by CALIMFAC
(Relative to No-I/M Baseline)

Near Term







NOx Original Program






2% Enhanced Program
(SB 1997 fully implemented)






5 Enhanced Program (plus Annual Inspection)






8 Enhanced Program plus Loaded Mode Testing







Sensitivity analyses show that the model is most sensitive to inspection frequency and inspection test type. These results, which are also summarized in Table I-l, indicate that relative to other program parameters evaluated for sensitivity, the implementation of an annual inspection program would result in the largest near-term improvement in HC benefits beyond those already achieved through the implementation of SB 1997. The model predicts that even with no additional improvements to the program, this change would produce HC benefits of nearly 30%, CO benefits of approximately 35% and NOx benefits of well over 20% within about five years of program implementation and HC, CO and NOx benefits of 22%, 32% and 8%, respectively, after about 20 years. The other significant program change that provided the greatest long-term improvement is the implementation of a loaded mode tailpipe test for 1980 and later model year vehicles (20% HC, 25% CO and 15% NOx soon after program implementation, with HC and CO benefits increasing to 23% and 33% in later years). Other program changes to which the model was sensitive include improving the performance of mechanics in identifying and repairing vehicle defects (19% HC, 29% CO and 8% NOx by 2010); and increasing the number of components included in the visual/functional underhood inspection (increases long-term NOx benefits of the program to approximately 8%). Removing the cost limit on repairs would, also have small but measurable beneficial effects on the effectiveness of the I/M program.

For questions regarding this research project, including available data and progress status, contact: Research Division staff at (916) 445-0753

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