Chris Quinn escaped serious injury from a fall by wearing a helmet.

Nobody knows what Sigrid Leirmo was thinking.

A little after 8:30 a.m. on Oct. 23, 1990, Leirmo settled her car into Lot 60 on the UW campus and released her bike from its rack. Sigrid hadn't cycled much as a child and was not a hard-core biker. When she bought her first mountain bike as an adult, she'd even practiced in quiet areas before venturing into traffic. Her bike was mostly a practical way to get from her car to her bacteriology lab.

She didn't make it. Pedaling onto Willow Drive -- which at the time accommodated both cars and a 10-foot-wide bike path -- she rounded a corner and headed up the slight grade, where she encountered Gerald Hall riding in the other direction. "Both zigged, both zagged, both zigged again," UW-Madison Police Lt. Gary Moore told the Wisconsin State Journal. About 50 feet from the entrance to Willow Beach they collided with force enough to bend Leirmo's frame. She went flying, hit her bare head, went dark.

Sigrid Leirmo never regained consciousness and was declared dead at UW Hospital at 9:50 that morning. The coroner determined that, had she been wearing a helmet, she would probably have lived. Hall, helmeted, cut himself up and lost a tooth, but walked away.

Leirmo's father and cousin collected her car afterwards. On the passenger seat was Sigrid's helmet. "That was a real shock to me," recalls her cousin Robert Rand. Her father, David Leirmo, remembers two helmets in the car. Family, friends and colleagues all remarked that she was a cautious type, a helmet wearer. Why hadn't she put it on?

I have never been able to shake the wild random circumstances of this crash. Such a small decision, such a minute deviation from her norm, and suddenly Sigrid Leirmo is gone.

I've been thinking a lot more about it lately because I've noticed more and more bare-headed Madison bikers. I can't prove it -- more on that later -- but I've lived directly on or within sight of some of Madison's most traveled bike paths for nearly two decades. I'm often out during commute time, and suddenly it seemed like helmets were in decline. There was a new generation of younger, hipper urban bikers who seemed to bare their heads with rebel pride. Then I saw familiar faces who had shed their helmets. B-cycle riders cruised by, usually without helmets. I'd see families out, the kids helmeted and the parents unprotected.

When my kids and their cohort became teens I started seeing young commuters biking to school -- yes! -- with helmets dangling on their handlebars. No! Sometimes I scolded them directly. Sometimes I ratted them out.

To be clear, I'm a helmet wearer, and I'm comfortable cautioning kids I know. I am not interested in mandating helmets, at least for adults. I understand the allure. It's fun to think we're in Amsterdam or Copenhagen. A helmet can feel awkward and artless, and the wind in your hair is a liberating, sensual feeling. So I'm told.

But I'm also perplexed. In the last decade our long-running wars have given us a painfully public knowledge of traumatic brain injury. We know more about concussion problems that could transform the sporting scene. New imaging science has wrapped probing fingers around our gray matter, giving us a wondrous view into our brain's strength, plasticity and ultimate fragility.

I love that more people are biking. But why do we choose now to stop wearing helmets? What would Sigrid think?

The case for and against

We are in a golden age of cycling. Gorgeous machines regularly spin by, sleek frames in eye-popping, candy-coated splendor. With matching wheel sets! Some employ technology that, on a larger scale, might take you to the moon. Others are simple triangles and circles of steel with 30-year-old components.

And after decades of struggle, bicycles are finally winning belated recognition as important players in our transportation economy. Having secured some share of our massive public roads expenditure, infrastructure for cyclists is on the rise. Intrepid riders and entrepreneurs keep stretching the boundaries, pushing bikes into places you never saw them just a decade ago.

Helmets remain an area of active debate. Statistically speaking, there's no data to support my observation that helmet use is in decline. I conducted two surveys of my own. Sitting near the UW Hospital one summer afternoon, I found about 60% of commuters were helmeted. Women were more likely to wear helmets, and virtually all of the riders without helmets appeared to be under 30. I also surveyed the bike racks at the Orton Park festival in late August, comparing the number of bikes to the number of helmets. There, about 40% of riders were helmeted.

Two different locations, two different populations, and all I really learned was just how hard it is to come up with useful data. Scouring academia didn't provide any recent findings, either. And of the dozen people I spoke to while reporting this story, some thought helmet use was up, some thought it was down, others thought it remained the same. They reported strong helmet cultures in New York City and no helmet culture in Davis, Calif.

Chuck Strawser, UW-Madison's bicycle and pedestrian coordinator, recently visited Davis, a city with bike paths everywhere, to observe its cycling safety culture. California requires bike helmets until the age of 18, but practically no one older than that wore a helmet in Davis, says Strawser. In a full week he saw only three. Two of them were road bikers obviously returning from a spin far outside the confines of the city. The other was his counterpart at UC-Davis, who was "trying to set an example, and failing."

Strawser normally wears a helmet, and even pays hundreds of dollars for the latest in headgear for him and his family. But in Davis, for the sake of his experiment, he went native. "It took me half a day to habituate to not wearing a helmet in Davis," he says. "Because nobody was wearing one and we all felt safe."

A lot of studies have been done trying to tease out what makes bikers safe. Sometimes helmet use is shown to have a positive influence. Other research shows that helmet laws and even militant helmet advocacy tend to discourage cycling. Investments in biking infrastructure have a clear benefit. But Strawser notes that some of the most detailed analysis finds that bicyclist safety is most closely tied to the number of cyclists on the road.

"An argument can be made that mandatory helmet laws and/or militant advocacy of helmet use actually make the cyclists that are still on the road less safe because they are not surrounded by as many other cyclists," he says.

Other research suggests that in aggregate the health benefits from being active are overwhelmingly more positive than the health detriments from cycling injuries.

It's an intriguing case for building a bike culture. But don't confuse population benefits with the personal ones.

"Of course this doesn't apply to individuals," says Strawser. "If you're the person that gets hit by a car and you weren't wearing a helmet, then obviously it all goes out the window."

The hair factor

The first time I met Emlyn Agnew he couldn't walk or talk, but he was aggressively working his bouncy seat dangerously close to the edge of the kitchen counter. He might have benefited from a helmet even then. It wasn't long before he was wearing one. Both of his parents bike, and helmets were mandatory.

But Agnew was also the first kid I knew who began to ditch his helmet while his parents weren't looking. I warned him, told him I'd keep it between us as long as he had it on the next time. I wasn't the only scold: A perfect stranger once passed him and nodded at the helmet on his bars. "Are you planning on putting that helmet on your head as you fall?" he asked.

His parents pleaded. "They were on me about it always," says Agnew, now 19 and a freshman at Earlham College. It didn't matter what they thought. He's a little embarrassed to admit it now, but it was about the hair.

"In my head I think that helmets just didn't look at all cool or attractive," he says. "I had put my hair up in that little rhino horn thing, and if I wore a helmet it didn't really work. It was a direct collision of interests there."

Then, one night last summer, he was riding home late, down Oakridge Avenue. No hands, no helmet and, ultimately, no balance. The fall: "I managed to sort of turn such that I landed on my back, and my head bounced on the concrete once," he recalls.

He lay there, dazed. "Holy shit, it's really that easy, isn't it? I think I'll get up and get a helmet tomorrow morning."

Head trauma

Dr. Lee Faucher, a trauma surgeon at UW Hospital, knows helmets are a hard sell. He and some colleagues once analyzed the hospital's accident data. Experience and intuition told them that helmets matter, but accidents are complex events that don't easily fit statistical models. They couldn't prove that helmets save lives.

But Faucher can tell you what happens when your head hits something hard. "All the energy is transferred to the inside," he explains. "The skull hits the object, and then the brain hits the inside of the skull," he says.

What helmets do is reduce the energy transferred to the brain. Like an airbag or a crumple zone, they slow down the crash.

The brain is a very complex electrical circuit, and a head injury scrambles communication between brain cells. "It just is not going to work very well," Faucher says.

Impact also disrupts the delicate network of arteries and veins, and bleeding can disrupt brain function. If the bleeding is bad enough, pressure builds within the skull, which decreases blood flow. Any oxygen deprivation leads to further dysfunction down the road.

UW neurosurgeon Josh Medow picks up the story from there. While a helmet is no guarantee, in his experience, most people wearing helmets "are fine and ready to go in a couple of days."

Recovering from head trauma, on the other hand, can be a significant challenge. Imagine not being able to balance a checkbook or not being able to read a newspaper article once and understand it. You might have weakness in a limb0 or a side of the body, or difficulty speaking. You might lose motivation, even moxie. Many trauma victims will gain a substantial amount of weight. Parts of you may never be the same.

Medow, too, knows the bike helmet data is hard to interpret. But he emphasizes that data from motorcycles show that even in highway speed crashes, motorcycle riders wearing helmets are much more likely to die of other kinds of catastrophic injuries and not head injuries. "The helmet really makes that huge of a difference," he says.

The latest technology

That leaves you in the hands of people like Chris Quinn, one of the sales and fitting experts at Machinery Row Bicycles. Quinn argues that the competitive riding atmosphere has become very pro-helmet, particularly among people who want to participate in recreational rides or races. Because competition rules require helmets, riders typically train with helmets too.

The rule saved him. He started racing at 17, and reluctantly donned the "bucket head" helmets available, but switched to the hipper Giro as soon as they arrived. He was on to his second model when, at age 23, he needed it.

He was just finishing up an exhausting training ride, pedaling slowly into his neighborhood, when a 3-year-old girl ran out from between parked cars and blindsided him. She knocked him in the hip and, standing out of the saddle, he flew headfirst over the handlebars.

As it happened, there was a motorcycle rally in the region, and the hospital had recruited a head trauma specialist for the occasion. The doctor told Quinn he had a concussion and scolded him for not wearing a helmet. When Quinn produced his Giro, the doctor revised his story, and told Quinn he should have fractured his skull and broken a cervical vertebrae that might have paralyzed him from the neck down.

What left a bigger impression was 72 hours of double vision. "What if I wasn't wearing a helmet?" he asks. "That's the chill factor right there that makes you want to look for better product."

And he's happy to report that helmets are getting better, with a wider variety of styles and colors. Gone is the head bucket, the reflector-taped Bell helmets that were impossibly square for an object designed with aerodynamics in mind. Now helmets can project an extreme sport image, alien headgear, or even a Harley vibe. "You're not getting an obvious my-mom-makes-me-wear-this-helmet theme," says Quinn.

But as an investigation published this summer in Bicycling magazine details, helmet standards are also old and limited in scope. Currently bike helmets must meet a single test: They are strapped to a five-kilogram weight and dropped upside-down on an anvil from a height of two meters. Motorcycle and ski helmets are far more advanced in absorbing the variety of impacts that real-world crashes involve. And while these new technologies are making their way to the marketplace, it's not clear how or even if regulations and standards will evolve to help cyclists make informed choices.

Quinn has watched improvements over the years, and acknowledges the limitations of current technology. A first adopter, he's looking forward to the next generation of helmets. "I'd like to see an arms race in the industry," he says.

Helmet culture

I flagged Kim Benson down on the bike path because of the contrast: dreadlocks and a helmet. When she was growing up, her parents always made her wear a helmet and closed-toe shoes when riding. During her first year at the College of Visual Arts in St. Paul, Benson grew dreads. "Then the helmet didn't fit," she says, and for years she resisted getting another. "I did kind of feel this looks so bad wearing a helmet."

There wasn't really a helmet culture among her peers in the Twin Cities. Partly it was aesthetics; partly it was comfort. "When you're hot and you're sweating, that breeze feels real good on your head," she says.

A co-worker's run-in with a truck changed her mind. She suffered a brain injury and major complications.

Now Benson, in her second year of painting in the UW's MFA program, has just a few remnant dreads. She misses them, but one of the benefits is that her helmet fits better. "I'm biking more, and just more aware of biking generally," she says. "I bike almost everywhere."

And that's the goal of bike advocates of all stripes, from Strawser to B-cycle's Claire Hurley. "We just want to make it really easy for anyone to get on a bike in Madison," Hurley says. Their research shows that 30% of their trips have replaced single-occupancy car trips, a boon to pretty much all of us.

Bike-sharing programs have recently come under fire for not offering helmets. Hurley says it's difficult to rent helmets because once a helmet is used by a consumer, it's hard to guarantee its integrity. Drop it from a height of two feet and it may be compromised. Single-use helmets are being developed, but aren't quite ready. In Madison, at least, members of the Monroe Street Merchant Association have banded together with B-cycle to sell $6 helmets.

"There is no doubt that helmets are important, and we always encourage our riders to wear a helmet if they have one available," says Hurley. Meanwhile, she happily reports that of the 136,179 B-cycle trips from June 2011 until Aug. 24, 2012, only two have resulted in reported accidents, with no injuries.

A terrible loss

Should you wear a helmet? That's your call, and I'm guessing statistics won't change your mind. Using science and conjecture we could cook up some plausible number to show Sigrid Leirmo's death was one in a billion. And I think we can agree that if humans ever began planning to avoid that level of risk, we'd probably fade away.

But I've never been able to shake the image of Leirmo's death. I'll always wonder what she was thinking.

"Such a nice day," her father explains, 23 years later, a hint of sadness in his voice. Her cousin Robert Rand remembers that she had a presentation that day, and figures she was just preoccupied. But might she have been concerned about her hair?

"Oh, no. No, no, no!" exclaims Laurie Hall, her best friend, laughing a bit. Then, quietly: "I think she was in a hurry."

Hall still cries when she talks about Leirmo. They met working at a Willy Street drop-in center where they cared for homeless children. They kept in touch through Hall's grad school years in Iowa, and the end of Leirmo's first marriage. Leirmo became godmother to Hall's first daughter.

Like many, Leirmo had fumbled a bit through her twenties. "She was just finally getting to a place where she was really sure about what she wanted to do," Hall says. "I think she felt good about where she was going. It's a terrible loss for everyone. Her dad was so proud of her. It was just unbearable for everyone."

A few days after the crash, Rand took Sigrid's parents to visit the crash site. They wanted to see it, to get a sense of closure. The crash had knocked a blue stone from Sigrid's wedding ring, and they hoped maybe they'd find it. They looked under the autumn sky, among the blowing leaves, guided by the chalk marks that showed where she fell. They never found it.