Sure, but cars aren't subsidized at $18 dollars per trip. The cost to the government of providing and maintaining roads is on the order of several cents per vehicle mile traveled.
For BART, that number is:
$1.07B operating expense / 892.5M million passenger miles traveled = $1.20 per mile. Then subtract out the fare per mile and the government is paying $0.84/mile for people to ride BART.
For BART, that number is:
$1.07B operating expense / 892.5M million passenger miles traveled = $1.20 per mile. Then subtract out the fare per mile and the government is paying $0.84/mile for people to ride BART.