To understand what happened next, it helps to know a couple of things: Ninety percent of the vehicles on Vietnam's roads are motorbikes and until Dec. 15, 2007, fewer than 10 percent of riders wore helmets. The second is that, in this rapidly developing country, traffic controls are - like the Pirate's Code in Pirates of the Caribbean - more what you'd call guidelines than actual rules.

"It happened very quickly," Moxham recalls. "A young woman on her motorbike was knocked from behind. She dropped her bike, fell off, hit her head on the road and died. Just like that." She shakes her head. "Dead."

During the previous 18 months, Moxham says, she'd seen traffic in this capital soar, and along with it, a mounting carnage. Government estimates put the death rate at 30 a day - like losing a 737 planeload of passengers every two weeks.

"We were seeing dead bodies everywhere," she says. "The traffic here is so awful and it has become exponentially worse within less than a year."

On Dec. 15, however, Vietnam enacted Resolution 32, its mandatory helmet law.

Since then, officials report a drop of 20 to 30 percent in traumatic head injuries and deaths from motorbike accidents, making it one of the world's most successful public-health initiatives in years.

That good news arrived almost to the day that, halfway around the world, the University of Pittsburgh released a report showing that since 2003, when Pennsylvania repealed its mandatory helmet law for motorcycle riders, head injuries and deaths have risen sharply: an estimated 32 percent increase in head-injury deaths and 42 percent rise in head-injury-related hospitalizations.

"The countries that adopt and enforce helmet legislation reduce injuries and deaths. And the states that repeal them see an increase," says Etienne Krug, director of Violence and Injury Prevention and Disability for the World Health Organization in Geneva, Switzerland. "It's just a fact."

As any casual observer in Hanoi can see, the Vietnamese are obeying their new helmet law at extraordinarily high rates. Some put the figure at 90 percent compliance.

"I started wearing one when the government insisted on it," says Bui Thi Thao, 23, a graduate of the Hanoi Open University with a degree in hotel management. "At first it was uncomfortable, but now I'm used to it."

Shops display the utilitarian headgear in every style and shade from camo to cotton candy, stacking them like coconut shells. Men here aren't afraid, by the way, to sport pink ones. But women who ride their motorbikes to work in spiked heels and tailored suits have taken the stylistic lead. They now accessorize with sunhat-style brims in pastels and lace, florals and Burberry plaid that slip over their helmets and can be changed daily to match their outfits.

Numerous nonprofits and nongovernmental organizations have been working for years throughout Asia and Africa to promote the use of helmets. But just as in the United States, there has been resistance because riding bare-headed simply feels better.

In Vietnam, climate complicates the problem. Heavy motorcycle helmets were dubbed "rice cookers" for reasons that are drenchingly obvious to anyone acquainted with the summer's heat and humidity.

"The goal was to get as many helmets on as many heads as possible," says Terry Smith, a member of the board of the nonprofit Asia Injury Prevention Foundation. "The victim in many of these accidents is the wage earner. So when he gets injured, the social cost is enormous."

Smith, who has a doctorate in biomechanics and works for Dynamic Research Inc., a helmet testing lab in California, helped develop a helmet that was lightweight and ventilated enough to suit the country's tropical conditions.

These helmets are now being produced in a factory on the outskirts of Hanoi and sell for about $10 - about the same as the cost of the fine if police catch you without one. For the typical citizen, whose annual income is less than $800, those fines provide a powerful incentive to wear - if not necessarily properly fasten - the headgear.

During the first few weeks men, women and children all wore helmets, even though the law requires helmets only on children over 14. But then rumors began circulating that even the smallest helmets are too heavy for a child's small neck to support.

Outside Huu Nghi Viet Duc hospital, a large glass-enclosed bulletin board serves as a warning of the risks of overloading motorbikes, and neglecting to put helmets on children. It contains large photos in full, garish, bloody detail showing children with crushed skulls and limbs from motorbike accidents.

The prevailing wisdom, however, maintains that children are safer without. So now mothers on motorbikes dash through the city with bare-headed babies lying across their laps. And parents, both wearing helmets, sandwich their toddlers between them, covering them with tentlike nylon ponchos when it rains - but never a helmet.

Full-face motorcycle helmets surely would be too much for a child, said Smith. But the benefits of lightweight, kid-size versions, he says, "far outweigh any problems that may be associated with neck strain."

"The arguments we're hearing are 'I'm not riding fast' or 'I'm protecting my child,' " said Smith. But as the accident that Rose Moxham witnessed shows, you don't have to be moving at all to suffer a fatal fall.

"You're falling from five or six feet up," said Smith, "onto a flat surface. Before parents can do anything, it's over. Accidents occur quickly."

Not all victims, of course, are on bikes.

To cross a street in Hanoi requires an act of faith. With over 60 percent of the nation under age 30, a noticeable contingent of riders is fueled by youthful impatience and testosterone. They text while driving, whip by pedestrians within a heart-attack's breadth, and weave in and out of traffic.

Pedestrians learn to cross streets steadily and slowly, while the honking flocks of motorbikes approach, part, fly around them, regroup and carry on.

The chaotic courtesy impressed Seymour Papert, a Massachusetts Institute of Technology professor and expert in artificial intelligence. Papert was at a conference in Hanoi in 2006. On the last day, walking from his hotel to the meeting, he was talking to a colleague about creating a mathematical model of Hanoi's traffic when he was hit in the chest by a motorbike.

After weeks in a coma, he was airlifted back to Massachusetts. Several surgeries and months of rehabilitation later, he now lives in Maine, severely debilitated.

In America, some states that repealed helmet laws have reinstituted them after watching rates of death and injury from head trauma rise. The Pittsburgh study, however, has not moved Gov. Rendell to consider changing course.

"The governor understands the statistics," says Chuck Ardo, Rendell's press secretary. "And he encourages all motorcycle riders to wear a helmet. But he believes it is a matter of personal choice."

As Pennsylvania's ardent anti-helmet groups have maintained, the protection helmets provide is no guarantee of safety in all circumstances.

For example, in Hanoi two weeks ago, Moxham spent the afternoon at the bedside of her friend, Hania Galan.

Galan, a young Canadian artist and former teacher at the U.N. Independent School, was found by the side of the road on June 9, her motorbike by her side. Her friends have heard that a motorbike cut her off and she swerved into a concrete barrier.

"She's been in a coma," Moxham said. "Someone found her and dropped her off at the hospital. She was wearing a helmet."

An e-mail circulated among Galan's friends last week reported that she opened her eyes in response to her name. Her family was joining her, and there were plans to have her flown back to Canada as soon as possible.