IRL pontoon bridge also like a dumb - we can not swim under them. only between pontoons.
it is step one
and step two - who allow you to drive over steel construction? we can park our loot-bike within our tank-house?
it is a construction and car will smash construction to junk metal, if it is a construction, or push them if construction is like a car.
what is a bridge from the point of view of a game-engine?
usual bridge it is a… damb. i.e ground line with pave over it and fences by the sides of this road. or maybe not a damb, but river - it is a fake, and it is not a river but chain of a lakes between “bridges”.
i think - if we start to program bridgebuilding feature for game engine - we can have a hawl cars because we teach engine to overlap one tile over other. as a bug-feature, i guess 
but this is a hard work.
and in passing we should rewrite the procedure which calculates how many different freight is capable to transport this or that car.
for packages in a luggage carrier it will be necessary to conduct calculation not only of the volume but also weight.
then it will be possible to talk about несущесй abilities of bridges on which it will be possible to drive the car.
or to forget about bridges.
the bridge will give us the chance of use of long trailers and semi-trailers.