REGGIO CALABRIA, Italy — Italy’s A3 highway, begun in the 1960s and still not finished, starts outside Naples in the ancient hill town of Salerno and ends, rather unceremoniously, 300 miles farther south as a local street in downtown Reggio Calabria.

Along the way, it frequently narrows to two lanes, with an obstacle course of construction sites that have lingered for decades. Perilous, two-lane bridges span mountain ravines high above the sea, while unlit tunnels leak in the rain — and occasionally drop concrete and other building materials onto passing cars.

Nothing embodies the failures of the Italian state more neatly than the highway from Salerno to Reggio Calabria. Critics see it as the rotten fruit of a jobs-for-votes culture that, nurtured by the organized crime that is endemic in southern Italy, has systematically defrauded the state while failing its citizens, leaving Calabria geographically and economically isolated.

The highway is also a symbol of what some Northern European countries say they fear the most about the euro zone: its development into a welfare system in which they are expected to support a sluggish Southern Europe, where grants and subsidies too often vanish in graft that the governments appear unable — or unwilling — to prevent. And it helps illustrate how the financing has yielded relatively little of the productive investment that might now be helping Southern Europe as it tries to climb out of an economic ditch.