There are some land locked major cities where it is a very sensible use of public money. But the urge of the next 50 cities in size to latch onto the Federal teat of Mass Transit money is often a boondoggle for airport authorities and trade unions that local politicians want to pay off with government projects (pork) that they can bring to their local economies.
Often, short-sightedly.
Adding an extra lane merely funnels more traffic onto the connecting roads and streets that haven't (and likely can't) been expanded. It also increases the burden on limited parking availablity. The urban centers of most major cities are "landlocked", that's why they're "cities" -- the land has been fully developed and availability is at a premium. Mass transit is the most sensible means of moving people around in these densely populated areas. Yes, including commuting from/to the 'burbs and out to the airport.