Transportation funding is a complex web of money, taxes, and fees that is determined and handled at all government levels. Ostensibly, the rules are set up to assure that tax funds will be distributed equitably and will be spent efficiently, and that the requests for government funds will be limited to what is actually available. But there are other reasons as well, that generally involve pleasing one group of constituents at the expense of other groups – groups being transportation users, or taxpayers, or manufacturers or many others – the list is endless.
To put together a new transportation project, or to keep past projects running, the proponents must understand what they must say or do to draw funds from the “buckets” or accounts in which they are held. The “spigots” through which the funds flow from the buckets are complex combinations of rules of approval.
In the nine-county San Francisco Bay Area, funding is largely controlled by the MTC (Metropolitan Transportation Commission). MTC provides some helpful information on funding at https://mtc.ca.gov/our-work/fund-invest.
A quantitative overview can be obtained from some graphs used by MTC in public presentations. They show declining purchasing power of State and Federal fuel taxes, expected increases in population and travel, and a breakdown of future Bay Area expenditures on transportation.