Wouldn’t it be cheaper to build a sidewalk on the bridge rather than build a separate bridge for pedestrians?

 Making the highway bridge narrower will reduce theamount of structural steel used by about 200 tons, concrete by about 650 cubic yards (1,300 tons), and excavation by about 10,000 cubic yards of earth. This will save about $1,500,000 from the cost of the highway bridge. The cost of adding a cable supported pedestrian bridge below the highway bridge is also estimated to cost about $1,500,000. The cost of building a sidewalk on the bridge and the cost of providing a separate cable supported pedestrian bridge are essentially the same. See Question #1 above for reasons that make a separate bridge a better choice than adding a sidewalk to the bridge.