Suicide bombers in cars packed with explosives lurk at on-ramps, waiting for American convoys or other targets.

Insurgents in cars with darkened windows mingle in traffic, then lower windows for bursts of machine-gun fire. Disguised as members of a road crew, they bury daisy-chained artillery shells beneath the roadway, then trigger them with garage-door openers and cellphones.

But it took Italy's fury over the shooting of one of the country's top intelligence agents earlier this year for the American military command to acknowledge publicly that driving the airport road is a form of Russian roulette. "Route Irish is commonly referred to as 'the deadliest road in Iraq' by journalists, soldiers and commanders," the command declared recently, in a report that underscored the stresses affecting the American platoon that fired on the Italian agent's car.

Route Irish is the military code name for the expressway arcing eastward from what was once Saddam International Airport, flanked by mainly Sunni Arab neighborhoods like Amariya, Hamra, Jihad and Qaddisiya that were strongholds of support for Saddam Hussein. Like neighborhoods across the Tigris River from Mr. Hussein's seat of power in the Republican Palace, these suburbs - some little more than slums, others thick with palm-shaded mansions - were populated with Hussein loyalists by design. Ever alert to potential assassins, the dictator, a Sunni Arab, built Sunni suburbs at strategic points around the city to shield him from attack.

When American troops captured Baghdad, these districts became insurgent hide-outs. And from this grew one of the war's grim ironies. Two years ago last month, the airport road was the last leg of the invasion route for American tanks, which set out from their first Baghdad foothold, the airport, to seize the Republican Palace. Now, the palace compound, renamed the Green Zone, is home to the American Embassy and the new Iraqi government, and the airport road has become a totem of how embattled America's presence in Iraq has become.

The insurgency has proved to be alarmingly dynamic, with shifting tactics that have earned American commanders' respect, as well as contempt for the rebels' seeming indifference to the fate of the civilians, who have been their most numerous victims. But one constant has been the vulnerability of the airport road, a journey so loathed, particularly by foreigners, that it has become their personal bellwether for the state of the war. Most of those killed on the road have been Iraqis, innocents caught up in attacks aimed at foreigners. But at least 10 Westerners have been killed since mid-April, including Marla Ruzicka, a 28-year-old volunteer from Lakeport, Calif., who had lobbied Congress for American aid to Iraqi and Afghan casualties of American military strikes. On April 16, a suicide bomber careering from an on-ramp detonated amid traffic bottled up behind a Humvee convoy, incinerating Ms. Ruzicka's car and killing four people, including two Iraqis.

Four days later, three other foreigners died in a machine-gun attack a mile closer to the airport. A survivor of that incident, James Yeager, a security contractor who was previously an undercover narcotics detective in Camden, Tenn., said in an account posted on the Internet that his security convoy was halted half a mile east of the airport by an American military unit dealing with the aftermath of a roadside bomb.

Mr. Yeager said he noticed a parked car that appeared to be an insurgent lookout - with an Iraqi inside talking on a cellphone - on a neighborhood road parallel to the expressway. In minutes, a white sport utility vehicle with tinted windows drove down the side road, made a U-turn, then lowered its windows. A volley of machine-gun fire killed three contractors, an American, a Canadian and an Australian. The S.U.V. fled.

"I wanted to kill the terrorists, but nobody had told me the direction, description or distance," Mr. Yeager said.

So feared has the route become that an American security company announced earlier this year that it would run an armored-car taxi service to the airport from the Sheraton hotel on the Tigris's east bank. The cost, one way, per person: $2,390, probably the world's most expensive airport cab ride.

But whether the road is traveled in a patrolling Humvee, an armored car or an Iraqi family car or taxi, arriving safely is an occasion for celebration, and a sense that luck - or benign providence - has prevailed.

"As soon as I leave the terminal, or set out for the airport from the city, I begin reciting verses from the Koran to calm myself," said Mahmud al-Dulaymi, a 52-year-old airport maintenance worker who moonlights as a cabdriver on his days off. "I have no way of knowing when my turn as a victim will come, but as a Muslim I believe that I have a destiny, so I trust in God, and leave everything to him."

In the past year, American and British diplomats and visiting V.I.P.'s have been barred from using the road, and are flown to and from the airport on helicopter gunships, a 10-minute roof-skimming journey to the Green Zone. On a visit in February, Senator Hillary Rodham Clinton noted that she had driven along the road on a previous journey, in December 2003, and cited the change as a measure of how much security in Iraq had deteriorated.

Iraqi drivers, especially those working for foreigners, prefer to race down the six-lane expressway, veering away from on-ramps and keeping to the right lane to avoid bombs buried in the median. Some weave as they approach overpasses, to foil insurgents who might drop explosives from above. But just as often, the journey is an intimidating crawl lasting up to an hour, as traffic backs up behind American convoys - or the chaos caused by the frequent attacks.

Some American convoys carry red-lettered signs in Arabic and English warning drivers to stay back "100 meters" or face "deadly force." Procedures require turret-gunners to show a clenched fist to approaching drivers, throw water bottles or spikes, and fire warning shots before aiming at drivers. But often enough that it has become a point of anger across Baghdad, gunners have opened fire only to kill and injure Iraqis uninvolved in the insurgency, who were simply unfamiliar with the American procedures or too impatient to stay back.

American commanders have kept casualty tolls secret, apparently concerned for morale among American troops and other users of the road.

Before releasing the report on the investigation into the death of Nicola Calipari, a major general in the Italian military intelligence service, senior commanders often said the hazards on the road were exaggerated compared with other high-risk war zones across Iraq. "I don't notice anything out of the ordinary on Route Irish," Brig. Gen. Mark E. O'Neill, assistant commander of the Third Infantry Division, responsible for Baghdad, said days before the Calipari report appeared.

In the published report, the numbers of attacks on the road were blacked out. But with a few computer keystrokes, reporters recovered censored portions outlining the anxieties among American soldiers on March 4 when they fired on a Toyota sedan carrying Mr. Calipari and Giuliana Sgrena, an Italian journalist rescued minutes earlier from a month as an insurgent hostage. It said that there had been 17 suicide car bomb attacks elsewhere in Iraq in the week before the Calipari shooting, and that these had been passed to all American units in the form of "BOLO" - "Be on the Lookout" - messages.

The report said there had been 135 attacks on the road in the four months up to early March, including 15 suicide car bombs, 19 roadside bombs, and 14 attacks with rocket-propelled grenades. But even these figures may understate the threat. One report this year by a Western security company said airport road attacks had included 14 suicide car bombs in November and December last year alone, double the incidence cited by the Calipari report.

"The enemy is very skillful at inconspicuously packing large amounts of explosives into a vehicle," the report said. "When moving, these VBIED's are practically impossible to identify until it is too late." VBIED's - soldiers say vee-bids - is the American command's acronym for vehicle-borne improvised explosive devices.

In this high-stakes challenge that driving the road has become, home is a monument known as the Winged Man. The statue commemorates Abbas Ibn Firnas, an astronomer in Moorish Spain, who is said by legend to have built a primitive flying machine in A.D. 875 with which he made a gliding descent from a hillside outside Cordoba. His statue's spreading wings mark the point at which Baghdad-bound travelers bid farewell to the secured airport area, or airport-bound ones reach its safety.

One recent traveler, Ahmed Hashim, a 27-year-old Kurdish engineer, reached the Winged Man with relief. "When I'm on the airport road, I feel that there's only a hair's breadth between myself and eternity," he said.

Among American soldiers, assignment to airport road patrols is deeply unpopular. Some soldiers take care not to "battle lock" Humvee doors, a procedure similar to dead-bolting that makes the doors more secure but also slower to open. Many, like Iraqis, set out with prayers. But still, they agonize.

"The ones you worry about are the ones who don't get out of your way," said Staff Sgt. Daniel Staniewicz of Orlando, Fla., who commanded one recent Third Infantry patrol, speaking of Iraqi-driven vehicles. "There are so many vee-bids," he said.

Richard A. Oppel Jr. and Khalid al-Ansary contributed reporting for this article.