[jakebol]

Even after such infrastructure is built, a > 0 multiplier assumes the economic impact of the project would be greater than the lifetime costs of upkeep and maintenance of the infrastructure (and current and future taxes to support such infrastructure + interest on debt payments used in its initial construction). And excludes the opportunity cost of allocating that future capital to other projects. Ex. NY still throws money into the canal project, even though the canal has long since been economically unviable (but politically it is still viable and so it remains...).