Toughest commute in Iraq? The six miles to the airport.

Samson has been delivering supplies to US military bases for a year. It's a good business that sends him to Baghdad International Airport daily. He reads Psalm 91 before every trip.

Samson, a native of Madras, India, pulls out his dog-eared, hardbound Bible and reads his favorite verse. "A thousand shall fall at thy side, and ten thousand at thy right hand; but it shall not come nigh thee."

He prays because the four-lane, six-mile stretch of road leading from central Baghdad to the country's main airport remains one of the most dangerous stretches of highway in Iraq, if not the world. It functions as a critical supply line into and out of the country, traversed daily by US military convoys as well as Iraqi and foreign businessmen, journalists, and aid workers.

So why is this vital strip of concrete, which takes only minutes to travel, still so difficult to protect?

One answer is that despite numerous US military checkpoints, the insurgents know that this is a "target rich" route that is the heart of military and contractor supply routes, and they continue to challenge every security measure devised.

Last Wednesday, three foreign contractors were killed in an ambush on the road. Two more died a day later when a suicide car bomber exploded.

Aid worker Marla Ruzicka was killed a week ago when a car bomber targeting a convoy on the road near her vehicle exploded. And an Italian intelligence agent was shot in March by US soldiers when he was shuttling a journalist to the airport who had just been freed by insurgents.

"All aircraft come only for Americans," Samson says. "So ... for people going and coming from the airport, because everyone knows they work for the Americans" they're a target. "Everyone working for the Americans must be killed, this is what they think."

Rebel violence has surged in recent weeks as political momentum generated by the Jan. 30 election has waned. Twin car bombings killed 22 people Sunday, and more than 180 Iraqi security personnel have died in the past six weeks. Iraqi politicians again delayed naming a government Monday, prolonging a three-month political vacuum.

During that same period, the airport highway has been scarred with fresh scorch marks from car bombs and roadside bombs. The land surrounding the road is flat and lined with scrubby palm trees and brush. The US military has cleared the wide highway median to keep insurgents from hiding in the vegetation. But stopping car bombers from cruising the stretch, looking for armored cars or military vehicles, is harder.

"So many times [an attack] happened just before I got to the place or just after I was there," Samson says, declining to give his full name for fear of retribution. He says driving armored cars for protection can often makes matters worse. "You are [asking] people to hit you because they know we are working for the Americans."

The danger of the airport road also speaks to the wider problem of securing a country in the face of a dispersed and committed insurgency blended within the civilian population.Millions of cars traverse Baghdad's roads every day, and just a handful of them are carrying suicide bombers. For the Iraqi government and US forces, it's a needle-in-the-haystack problem with few practical solutions. There is limited US military manpower for adding checkpoints, but even if it was logistically possible, stopping every car on Baghdad's roads would bring the city to a grinding halt and make the airport journey even longer than it is now.

One security consultant working for a Western media company in Baghdad says he advises his clients to keep their eyes peeled, wear body armor, and avoid private security details "like the plague." These convoys are the biggest targets on the road, he says. But for high-profile clients, the consultant says, it's still better for them to ride in armored cars, because the assumption is they're already being targeted.

The airport road is a direct link to the US headquarters in the secured Green Zone. But rather than risk the road, US diplomats fly by helicopter from the airport to the Green Zone.

Many Iraqis who aren't going to the airport have to traverse part of the road to get around Baghdad. "If I have people with me and we take the airport road [the passengers] start praying from the entrance until the end and after we take an exit from the road they say 'hamdulillah alsalameh,' " says taxi driver Radee Taha, using the Arabic expression used after someone has returned safely from a trip.

Mr. Taha's brother was shot earlier this month when he was on the airport road, sending his car flipping several times. But it wasn't insurgents this time. He says the gunfire probably came from private security guards in an armored SUV. Jittery security details and soldiers can also pose a hazard on the road.

"After the accident of my brother I will not take the airport road again," Taha said.

• Wire services contributed to this article.