Road construction and maintenance can be paid off using taxes on various things(property taxes and gas taxes).
Mass transit relies much more on ridership. (NYC Subway is massively underpriced)
Also - road maintenance is highly dependent on road use. Biggest degrading factors for the westside bike path on Manhattan have been natural(weather and flora). I've been on bike paths that were paved in the 60ies and are still in great condition... without maintenance.