The geographic changes in ridesharing from 1990 to 2000 are quite pronounced. During the decade, only four metro regions of over 1 Million people had increases in carpool mode share and they were predominantly west of the Mississippi (Seattle, Phoenix & Dallas. Atlanta was the fourth). The map below paints an interesting story. While carpool mode share has been decreasing nationwide, the largest decreases have been in the eastern US. At the metropolitan level, the results are even more pronounced; of the top 10 metro regions with the highest carpool mode shares in 2000, eight are located in the US Southwest (CA, NV, AZ, NM & TX). The 10 metro regions with the lowest carpool mode shares were all in the Northeast (NY, MA, CT) and the Upper Midwest (OH, MI, PA). It is also interesting to note that the two metro regions that are frequently cited as examples of carpool success stories (San Francisco & Washington, DC) rank 16th and 21st respectively in terms of carpool mode share. Ironically, three cities often criticized for their reliance on the private automobile (Phoenix, Los Angeles & Las Vegas) have the highest proportion of carpool commuters in the US.
[Note that this analysis was limited to metro areas with 500,000 commuters or more]