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.
