This familiar expression implies that any decision to cross a bridge (or not) will best be made at the time we arrive at the bridge and (unless we are being difficult and impractical by trying to risk wading or swimming across the river) the 'crossing over' part of the equation is not negotiable.
That this commonly held truism holds up for most events we are thinking about in our journey through life, seems not to be in question.
While all this is obvious, the obvious is not always the only answer.
What about those who are traveling along the river? They are coming to a bridge too. But they will not be thinking or speaking sanely if they refer to the bridge as something they will have to 'cross' when they come to it.
They will have to go under it when they come to it, or the boat will crash. Thus the expression is not universally applicable.
Those who are traveling along the "road more travelled" need to be careful not to think of a bridge as something we have to all 'cross over'.
Those of us traveling by another means (perhaps by water) will have to think about crossing under the bridge when we come to it.
I wonder if the Mallecan's got that memo?