I don't know about the bridges specifically, but that the interstate system had an explicit national security purpose is not just well-documented, it's literally right there in the name of the law that created it: National Interstate and Defense Highways Act.
My issue with the bridge thing is that it's easy enough to blow up bridges, especially as you're retreating from the territory they're in, regardless of their location or construction. It could cost billions of extra construction dollars to unnecessarily site a bridge in anything but its obvious ideal location (e.g. the shortest possible span).
My understanding was that it was created to mobilize the military and transport resources across the continent, not to make it easy to cut off enemy routes.
