That being said, Bobby is probably correct as to the replacement method -- but with a potential "twist" -- building one new single-direction bridge (68' wide: eventually 3 12' lanes + double 12' shoulders + double 4" siderails) in between the two existing bridges, but substantially closer to one than the other. The old bridge closest to the new one would be kept intact for one direction, while the other is relocated on the new bridge. At that point the more distant bridge is torn down and replaced with a new structure identical to the first.
That approach could work, but it would come at a cost of taking out the boat channels. There are boat ramps in between the bridges at Exit 121 and 127. Farther East near New Orleans the I-10 and I-55 bridges over the Maurepas Swamp have accommodations for boating and fishing. It would be controversial to build a new bridge structure through much of the middle of that channel.
Considering what it will cost to replace any of these existing bridges, building new structures only wide enough to add a third lane seems like not nearly enough of a benefit in return for all the cost and inconvenience that will occur when the bridges are replaced. I think the planners should consider building each replacement span 4 lanes wide.
Except, the idea here is NOT to build a new 6-lane viaduct in between the current ROW, but simply add a third lane and appropriate shoulder. You only need an additional 12 ft. lane in either direction and a 10' shoulder, plus an occasional "layby" at regular intervals for accident clearance or breakdowns.
In theory that would work - but consider this. The 22 mile long bridges are nearing 50 years old. Considering any massive expansion is at least 10 years out, it’s a better bet to go ahead and replace the two viaducts with a new span entirely that would have a much longer life span, up to at least 100 years
Exactly. The existing bridges are OLD. There is nothing visually attractive about these bridges to warrant saving them from demolition and replacement with entirely new structures built to modern standards.
Simply adding an additional lane onto aging bridges could open a big can of worms in terms of legal and regulatory red tape. These existing bridges do not comply with modern Interstate highway standards and modern highway bridge standards. Would a significant alteration to these bridges (adding a 3rd lane) require the entire structures to be
brought up to code?Very often it's just more simple to demolish the old, out of date bridge structure and replace it with a new bridge. Lately the trend with major Interstate highway bridge projects has been to build twin spans to increase capacity.