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State Road 18 Lower Tyrhhenian
Some roads in Italy still follow the paths of ancient Roman roads. The modern Strada Statale (SS) 7 is an example. The modern name is SS 7 via Appia -- the ancient Appian Way, perhaps the most historic road in all of ancient Rome. For our purposes, note the modern designation Strada Statale (State Road). This entry is concerned with another State Road, SS 18 Tirrena Inferiore (lower Tyrhhenian), the long and important road that runs the coast from Naples all the way to Reggio Calabria (marked in blue on the map). It once was the only road from Naples to Reggio Calabria, thus connecting the main urban centers of Campania and Calabria. It has since been superceded in terms of speed and ease of driving by another kind of road, the autostrada, in this case the A-3 (shown in green on the map). This entry is concerned primarily with the State Road and secondarily with the Autostrada, the two types of roads that carry heavy traffic over paved roads for long distances.
Depending on where you drive in the English-speaking world, terminology varies.
A single carriageway (British English) or undivided highway (American English) is a road with one, two or more lanes arranged within a single carriageway or highway with nothing in the middle to separate opposing flows of traffic. There will be a painted white line, usually painted broken or solid to indicate whether you are allowed to pass or not, respectively. There is usually open access to and from cross roads and there may be a short third "waiting" lane at cross-roads for cars to turn off onto these other roads. Road traffic safety on this kind of road can be very dangerous depending on conditions: heavy traffic, night driving, night-time illumination, inclement weather, driving too fast, visibility, in a hurry to pass, etc. SS 18 is such a road. Under perfect conditions, it is a joy -- right down the Tyrrhenian coast with the sea just off to the right. Yes, there are guard rails along the side that have reflectors at night. Is it possible to breach those? Yes. Has it ever happened? Yes. But under perfect conditions -- an open small car or a motorcycle -- a ride from Sapri (a few miles below the Cilento bulge), the last town in Campania south to Reggio Calabria is what everyone who likes to drive wants to drive: 220 km/ 130 mi of peace on the road.
The road starts up in Naples, however, so before you get to all that peace and quiet, you have do some detouring; to get out of Naples, itself, and then around Salerno, you have to take the sections marked dir (for diramazione/branch), so SS18 (dir) means that you are still on SS18 (but this will get you around the inner-city and out the other side a lot faster. But after Sapri it's clear motoring. This whole modern set-up of State Roads goes back to 1928 and the Fascist reordering of the road network in Italy. The original "stops" of the SS18 were Naples, Torre Annunziata, Salerno, Battipaglia, Rutino, Vallo, Torre Orsaia, Sapri, Paola, Sant'Eufemia, Nicastro, Monteleone, and Reggio Calabria. That caused and still causes some problems because there is no single administrative body for the whole affair. You have a dozen different city halls trying to decide whether to repave this, or fix the lighting at km-number such-and-such, etc.
"Is there a problem, officer?"
This really is the E45 in northern Norway.
On the other hand, a limited-access road, known by various terms worldwide, including limited-access highway, dual-carriageway, expressway, partially controlled access highway, even the odd-ball turnpike (!) is a highway or arterial road for high-speed traffic, and is generally termed freeway (US) or motorway (Brit). (As noted, it is marked in green on the map.) It has limited or no access to adjacent property, some degree of separation of opposing traffic flow, use of grade-separated interchanges (that is, to exit you go out, up and over), prohibition of some modes of transport such as bicycles or horses (and pedestrians!), very few or no intersecting cross-streets and they may charge a toll. They may offer restaurants and other amenities along the way, including garages for mechanical repairs and even places to park your camper, bus or whatever "live-in" vehicle you have where you can spend the night. The A3, the autostrada is really the A3 only between Naples and Salerno. The official European designation is the E45 and is officially a single road from the very top of Norway (image) all the way to Reggio Calabria and over to Sicily to finish at Catania, Siracuse and Gela. It, too, started in 1929; the stretch from Naples to Salerno was the first autostrada in Italy. These autostrade are sort of self-sufficient, self-sustaing entities unto themselves. I imagine you could take your whole vacation on one of them without ever getting off, just cruising up one side and down the other, over and over, like the Flying Duchman sailing under a curse.