At one time, there was a high degree of confidence that ridesharing measures would make a noticeable difference to the number of vehicles on the road and total VMT. This confidence has largely dissipated. Carpooling has declined nationally (according to both major sources, 1980 and 1990 Census data and 1983 through 1995 Nationwide Personal Transportation Study data). Many States have focused on building HOV (high occupancy vehicle) lanes as a major component of their highway construction projects. Yet because of underutilization, New Jersey has converted some HOV lanes back to solo driver use, and other jurisdictions are considering similar steps.
A major debate is whether ridesharing can be boosted via mandates. The pioneering move in this direction was the South Coast Air Quality Management District's (AQMD) Regulation XV in metropolitan Los Angeles, introduced in 1988, which became the model for the ETRPs (Employee Trip Reduction Programs) adopted in the wake of the Intermodal Surface and Transportation Efficiency Act (ISTEA) of 1991. The key idea of Regulation XV was to require firms with more than 100 employees to implement ridesharing programs that would raise employee-vehicle ratios to 1.5 from their initial 1988 level of 1.15. The program must be deemed a failure. In the first year of operation, the average employee-vehicle ratio increased by only 2.7 percent (Giuliano, Hwang and Wachs, 1992), not very much given that those with favorable workplace and residence locations and times are likely to join first; ridesharing programs experience a diminishing marginal response, and not surprisingly any subsequent gains were minimal. In addition, dropout rates from carppoling programs are consistently very high. The Regulation XV program was very expensive, about $3,000 for each vehicle eliminated (Lane, 1993), three times as high as clunker retirement programs (typically, retiring a clunker will eliminate ten times as much pollution as getting a newer vehicle off the road). In view of the unpopularity of the program, its high costs, and shift in the AQMD policies to a more business-friendly stance, the regulation was put on hold in 1994 and has since been abandoned, although Regulation XV did trigger the introduction of ETRPs elsewhere in the US, primarily as part of the implementaion of ISTEA (the Intermodal Surface transportation and Efficiency Act of 1991).