Long-term decline in carpooling in the USA

Since the 1970s, billions of dollars have been invested in carpool lanes in U.S. metropolitan areas. Despite the availability of these facilities, carpooling has steadily declined from about 20% of commute trips in the 1970s to less than 10% now (see figure).

There is a good chance that some of the decline in carpooling is attributable to lower fuel costs. The average cost per gallon of gasoline in the USA was $1.22 in 1980 and in January 2018, it was $2.50. This seems like a significant increase. But when you adjust for inflation, the $1980 cost of $1.22 per gallon is equivalent to $3.92 in 2018. So, the real cost of gasoline in January 2018, was 36.5% less than in 1980.