Covered bridges were covered for a reason- they were made of untreated wood. The shed like cover kept rain and snow off the structural members, and made them last longer. When we began making bridges...
Since there are no covered bridges in the Southern states, I would think that the advantage is that it keeps snow off of the bridge during winter which makes it easier to cross and keeps it from...
An advantage to covered bridges is that they shield traffic from the elements (rain, snow, high winds, ect.). One semi-common disadvantage is that covered bridges tend to burn more easily than other...