Of note to this topic, have a look at
this very recent commit Standard, which permits elevated ways to be used over shallow water (with a depth equal to 1). This possibility may wish to be considered when implementing bridges.
Edit: One thought has occurred to me on the utility of maximum length restrictions, which had not occurred to me before: if I understand correctly, the view so far taken, I think, has been that there is no need to have different types of bridges with and without pillars, since pillars are simply added automatically where and if they are needed, and, since a bridge with pillars is usually theoretically unlimited in length, there is no need to have a length limitation for any bridge.
The difficulty that has just occurred to me with that is this: there is also a maximum
height parameter. Bridges with no pillars, obviously, have an unlimited height (as it matters not one whit to the structural integrity of the bridge how far the drop is provided that it is secured effectively on its banks). Conversely, bridges with pillars, while having a theoretically unlimited length, have a limited height. So, pillarless bridges have a height limitation, and pillarful bridges have a length limitation. Putting the two together is not the same as having a bridge with neither limit, since a particular type of bridge might need to be too long to be built pillarless, but over too high a gap to be built with pillars, making the gap uncrossable without using a different type of bridge.
See also
here for my extension request in relation to bridge heights.