If the bridge is medal framed (truss construction) it acts just like your car or a metal building, safer than many other places you can be. Most bridges only have some steel or concrete joists, that would offer little to no protection.
What Happens When Lightning Strikes a Steel Building?
July 23, 2019 by Steve Polston;
If lightning harms steel structures, then driving through a thunderstorm in a car would be a very unpopular idea. The truth is, the frames of pre-engineered steel buildings act as a “shield” that transfers most of the energy of a lightning strike down to the ground, leaving the objects and people inside them virtually unaffected. This phenomenon is known as Faraday shielding. The building itself becomes a Faraday cage that equally distributes the charges throughout its surface and keeps you well protected.