The ins and outs of a surprisingly frequent phenomenon.

A curious, if generally underappreciated, feature of a highway or a road is that it sometimes becomes a runway.

While such intersections of aerial and terrestrial transportation are not common, they happen with a scattered frequency you might find surprising. Last month, for example, the pilot of a single-engine Cessna, ferrying a traffic reporter in the predawn prologue to rush hour, made an emergency landing on the not-yet-congested New Jersey Turnpike. (Which exit, as one asks of things New Jersey? Exit 4, near Cherry Hill.) A week later, the pilot of another single-engine plane lost power and landed on a road just off Highway 50 near Sacramento, Calif., striking a car but causing no injuries. And, just last week, another single-engine with mechanical difficulty landed on U.S. Highway 6 in Spanish Fork Canyon, Utah.

There are no hard numbers on annual occurrences of airplane landings on highways or streets, but a troll through the Federal Aviation Administration's incident database shows that there tend to be more than a dozen such events in any given year (that the FAA knows about, at least). The events range in nature and geography. Mechanical difficulty ranks prominently in the causative universe. But pilots running out of fuel ("fuel starvation," as investigators put it), whether owing to unforeseen flight complications or actual negligence, is common, too. One FAA report dryly refers to a plane that "landed on a public street to discharge a passenger." And emergency landings can take place on deserted country roads, residential neighborhoods, or bustling thoroughfares. As the FAA's Les Dorr, after looking through the database himself, put it to me in an e-mail: "Highway landings are rather more frequent than I would have thought."

It's not difficult to imagine why aircraft often end up on the road. First, there is the inevitable condition that some small percentage of the planes in the air will encounter engine trouble. In some of these cases, pilots can limp on to their original destination or find a nearby airport. Then there are the other cases. "Total engine failure imposes surprise destinations," writes William Langeweische in his book Fly by Wire. "If not Charlotte, then some other airport. If not an airport, then an unobstructed highway; or, in descended order, a large flat field, an especially large golf course, a forest, or in the extreme, a lake or a river for ditching close to shore." Suffice it to say there are far more roads than runways.

Second, there is the physical resemblance between roads and runways (indeed, the FAA has tracked cases where pilots mistook one for the other). It is not uncommon for runways to be converted into roads (e.g., "Old Airport Drive"), and, as any taxi driver in Singapore will tell you, the long, straight section of the East Coast Parkway close to the airport, bordered by large potted plants rather than trees, can be converted into an emergency runway. (The tale recalls the old and incorrect story that Eisenhower's Interstate Highway System was built with a proviso that one in every five miles be straight to serve as emergency military landing strips.) In last month's New Jersey landing, the pilot noted that the well-lit Jersey Turnpike was the only thing he could see. The two forms are so similar that in some cases, roads are even used as illicit runways: Consider the notorious case of Matthew E. Duke, the renegade U.S. pilot who was ambushed by Cuban soldiers on a highway outside of Havana, after he'd landed his plane there to pick up Miami-bound anti-Castroites.

Roads present problems, however. As Jerry Eichenberger notes in his book Handling In-Flight Emergencies, "roads look inviting because most of us are used to landing on strips of level pavement." But, he cautions, "highways contain more traps for the unwary than are obvious at first thought." Power lines, for one, running parallel to or across the road. Traffic signs, which planes have been known to hit. And even big, wide freeways have obstacles like overpasses and bridge embankments.

Indeed, while landing a large plane on a highway might seem easier than, say, landing one in the Hudson River, history suggests that's not necessarily so. In the two most significant cases of a commercial aircraft attempting to make an emergency landing on a road, the difficulties came not with the landing itself but with ground obstructions encountered thereafter. In 1971, a Pan International Airline BAC-111 flying from Hamburg, Germany, to Spain made a forced landing on the Kiel-Hamburg autobahn, but 22 passengers were killed when, as one report described it, "the front fuselage section was cut off as a result of an impact with a road bridge."