I/M programs have been less effective than expected in identifying high emitters and ensuring in-use compliance with emission standards. Other approaches, such as remote sensing and on-board diagnostics, show technical promise, but further testing of those techniques is needed. There is also a concern that remote sensing may be viewed as unacceptably invasive.
As emissions from LDVs and LDTs decrease and the focus on attaining the NAAQS for PM2.5 increases, emissions from nonroad engines and heavy-duty onroad vehicles are becoming increasingly important. New emission regulations for these sources are being implemented, but the long lifetime of these engines will slow the rate of penetration of the effects of these regulations into the fleet. Additional efforts in inspection and maintenance and in promoting retrofit and incentive buy-back programs are therefore needed.
The inclusion of content-specific requirements in the fuel provisions of the 1990 CAA Amendments (for example, oxygen in the RFG program) can limit flexibility to meet standards in the most cost-effective way for areas required to implement the program. The standards also make it difficult to adjust the program in the face of new challenges (for example, in the case of MTBE contamination of groundwater and surface water) and have a small (sometimes even negative) impact on the control of some air pollutants (as might have been the case with the use of ethanol and potential increases in RVP).
Growth in vehicle miles traveled, personal automobile usage, and popularity of fuel-inefficient vehicles (for example, SUVs) has offset a significant portion of the gains obtained from stricter emission standards on individual vehicles. With the exception of the conformity requirements of the 1990 CAA Amendments and subsequent related legislation, air quality managers remain unable to affect these societal and behavioral determinants of mobile-source emissions.