Trucks gayly painted with hearts and doves jam up at crowded wayside bazaars. Billboards advertise cell phones and advise drivers to keep their donkeys off the road.

It's not readily evident that this is probably the world's most dangerous highway, a prime target for Taliban insurgents attempting to sever a vital, 1,000-kilometer (620-mile) artery with ambushes, executions and roadside bombs.

Widely seen as symbolic of Afghanistan's progress and security, or lack of it, Highway 1 suffered a dramatic increase in bomb attacks in 2009, but also a marked improvement along a critical 90-kilometer (55-mile) stretch after U.S. forces arrived in strength.

"Last year the insurgents were very successful in interdicting convoys. They can't stage that type of attack anymore," says Lt. Col. Kimo Gallahue, who commands a U.S. battalion guarding the highway just south of Kabul. "Since August we've been ripping through the enemy. Mass matters."

The situation is starkly different as the highway veers farther south into the Taliban heartland. Overall, roadside bomb attacks have risen by more than 50 percent _ from 308 in 2008 to 469 last year. But 394 were discovered before they detonated, up from 254 the previous year, according to a command spokesman, Lt. Col. Todd Vician.

Since the U.S. invasion of 2001, this vital land link between the country's two largest cities has been hotly and violently contested. About 35 percent of Afghanistan's population lives within 50 kilometers (30 miles) of the Kandahar-to-Kabul stretch, giving weight to the notion that "as the highway goes, so goes the country."

Battered by war and weather, the road got a $250 million makeover five years ago, halving the 12-hour, 483-kilometer (301-mile) drive between Kabul to Kandahar which have the two largest NATO bases. The U.S., Japan and Saudi Arabia then followed with an overhaul of the stretch from Kandahar to the western city of Herat.

Taliban leader Mullah Omar has good reason to target the road, says Col. David B. Haight, commander of U.S. forces in Wardak and Logar provinces which adjoin Kabul.

"If you were Omar, wouldn't you want to attack the country's most strategic highway, an icon of commerce economic progress? He sees traffic on the road and he doesn't like it. He has tried to disrupt it but he can't stop it," Haight said.

"There's never a day off. That road is very critical," he says, noting that the U.S. military has intercepts from Omar to subordinates stressing the importance of the two provinces because of their locations along or near the highway.