The US Census Bureau has recently released data showing that the percentage of workers who car pool has dropped by almost half since 1980.
Factors contributing to the decline in car pooling are thought to be a greater number of people driving, companies being more spread out, Americans being wealthier across the board, and cars becoming relatively cheaper to own. Somewhat ironically, the frustration of the traffic will often drive people to commute alone, preferring to manage the stress of congestion by themselves.
According to the NY Times, the sharp decline has confounded efforts by urban planners, who over the years have tried to encourage the practice by setting aside highway lanes for car-poolers, as well as offering incentives like discounted parking.