Appendix II – Documentation of the Emission Benefit Estimates for the
Performance of IM on Light Duty Diesel Vehicles

Calculation Methodology – See EXCEL Spreadsheet for Numeric Results
and References

The preferred methodology for estimating emission benefits of increasing
the stringency of an IM program is to use the federal MOBILE6.2 model. 
However, the MOBILE6.2 model does not contain the capability to model IM
programs for diesel vehicles. The NJDEP Bureau of Motor Vehicle
Inspection and Maintenance (BMVIM) has developed the following
methodology for estimating benefits for an IM program for light duty
diesel vehicles (LDDV) that involves OBDII testing and smoke testing
components.  

The first step is to obtain estimates of  LDDV counts from the New
Jersey Motor Vehicle Commission (MVC) registration database quarterly
extract from December 2007 data. Then estimate the projected inspection
failures that will result from each of the new IM program components.
The projected failures for LDDVs were estimated by considering current
failure rates for gasoline vehicles undergoing a similar inspection
test.  The smoke test failure rate was for vehicles that failed for
smoke only, i.e., vehicles that failed for smoke and some other test
were not included.  In addition, the failure rates and test volumes were
adjusted to account for the differing age distributions between light
duty gasoline and light duty diesel vehicles.  Emission benefits were
then calculated by multiplying the total benefits from the current
analogous gasoline IM program by the ratio of the projected new LDDV
failures to the current gasoline vehicle failures. 

OBDII testing of Light Duty Diesel Vehicles: 

 

Adding these vehicles currently untested for emissions to the IM program
will result in an estimated 4,190 new inspections per year with an
additional 356 failures. This represents about 0.3% of the current
program OBDII failure rate. The true impact will be driven by technology
changes implemented by the manufacturers and diesel vehicle sales in the
ensuing years.  

Smoke testing of Light Duty Diesel Vehicles: 

 

Adding these vehicles currently untested for emissions to the IM program
will result in an estimated 9,399 new inspections per year with an
additional 9 failures. This represents an added 0.008% to the overall
emission failure rate. Potential failure rates for Smoke tests on
vehicles equipped with new diesel particulate filter technology is still
an unknown. The true impact will be driven by technology changes
implemented by the manufacturers and diesel vehicle sales in the ensuing
years.  

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