Whether the verb is "was" or "was deserted", the subject of that verb remains "The wooden bridge straddling the river".
It seems to me that the sentence is, in effect, an example of
In effect, in the examples, I've been using because in its guise as a conjunction: A is B because C did D.
We could use "because" as an adverb: A is B because of C (e.g. I am here because of you). Now "A is B" is
** - One has to assume that either the people eating (not waiting to be served or reading their menus) left the bridge long before those heading for their cars, or the cars are parked a long way from the restaurant, or I've completely misunderstood what "away back to" means.)
It seems to me that the sentence is, in effect, an example of
X because Y,
where X is the bridge being deserted and Y being that those who were once on or at it (no sniggering, please, YouKnowWho) are either eating or on the way back to their cars**,
In effect, in the examples, I've been using because in its guise as a conjunction: A is B because C did D.
We could use "because" as an adverb: A is B because of C (e.g. I am here because of you). Now "A is B" is
The wooden bridge straddling the river was deserted
So what is C? If we use a melodramatic example, it could be
(because of) the troll.
which has the advantage of not requiring a verb or part thereof (although the reader might add, in their mind, "living under it"). Or, to move back to the original:
(because) of being abandoned for the evening.
But the "being abandoned" is still attached to the bridge, not to those who have now left. Can we get away with having no verb after the first comma? I can't see how, but this is a near as I can get:
The wooden bridge straddling the river was deserted, everyone having left to eat dinner at the adjoining restaurant or return to their cars.
which is a bit of a cheat, because the verbs are back (eat and return); but I have inserted a present participle (if that's what it is) in an attempt at legerdemain. (As before, I've not attempted to capture the dialect/use of words of the original.)
** - One has to assume that either the people eating (not waiting to be served or reading their menus) left the bridge long before those heading for their cars, or the cars are parked a long way from the restaurant, or I've completely misunderstood what "away back to" means.)