I think you're right that "surface" is the key word here, as that's what your feet/wheels interact with and what ultimately determines whether the suitability of the bridge for travel (it's also the only thing that's trivially verifiable, we don't want to have to start taking core samples of random roads!). In this bridges case, there's an asphalt/tarmac layer across it's whole length: https://www.geograph.org.uk/photo/6512139