Beginning January 1, 1997, with the passage of Senate Bill 836, California work sites with between 100 and 249 employees were exempted from the South Coast Air Quality Management District's Rule 2202, On-Road Motor Vehicle Mitigation Options. Rule 2202 had required these work sites to reduce mobile source emissions by implementing an emission-reduction strategy such as scrapping old vehicles, participating in the Air Quality Investment Program, or implementing an Employee Commute Reduction Program (ECRP). To estimate the resulting shortfall in emission reductions, surveys of voluntary ridesharing at deregulated work sites were conducted in each subsequent year: 1997, 1998, and 1999. Survey results indicate that average vehicle ridership (AVR) at deregulated work sites has declined compared with the AVR while under regulation. However, the AVR over the 1997 to 1999 period has remained relatively constant. For those work sites that had been complying with Rule 2202 by implementing an ECRP, the future baseline AVR was assumed to increase annually at the rate of increase before deregulation. For work sites that had chosen the other two options, the AVR target remained at 1.5, the same as prior to deregulation. Using the difference between the target AVR and the AVR measured in the surveys, for each of the three options, the emission reduction shortfall was calculated. The shortfall increased from 1997 to 1998 but fell from 1998 to 1999.