Carpools and vanpools are a half measure. They are too ephemeral to be a permanent improvement. Fee funds should be used only to purchase transit passes, and definitely not to buy gasoline. The private automobile is already too heavily subsidized. Supporting transit helps many more people, at far less cost, and continues to do so far into the future.
Rail transit requires the least amount of land per passenger served.
Any private vehicle is rarely used to capacity, and is thus wasteful. Roads, by their very existence, destroy farmland and wildlife habitat, even when they carry no traffic. Solar Electric Engineering of Santa Rosa, CA, is already making them. I see no reason to continue subsidizing only our largest car manufacturers, who are deliberately waiting to the last minute to offer electric vehicles. We must stop subsidizing people who choose to live in an auto-dependent location, by funding carpooling and vanpooling.