Six days after the worst domestic terrorism attack since 9/11, Los Angeles will stage another of its celebrated CicLAvia events.

The three-times-a-year bike ride takes place in wide-open streets blocked off by the L.A. Police Department and city Department of Transportation workers. Like Boston's 26-mile marathon route, CicLAvia's 15-mile cruise from downtown to Venice will be lined with crowds almost impossible to vet.

It's a glorious concept, a noncompetitive romp with friends and family along boulevards that are otherwise rarely, if ever, car-free. It was inspired by Bogotá's similar Ciclovía street parties. But the contrast here — with the car capital of the United States partying to pedal power — makes CicLAvia all the more remarkable. It draws tens of thousands of riders each time it happens.

The big question is whether such sun-soaked parties are a thing of the past after three people were killed and 176 injured in dual bomb blasts near the Boston Marathon finish line Monday.

The true terror of the Boston attacks is that they exploited a vulnerable yet sacred spectacle — the open-air, unticketed sporting event. Blasts at or near bleachers, reportedly caused by pressure-cooker devices filled with ball bearings and other shrapnel, were comparatively amateur yet wholly effective at rocking the American psyche at a time when domestic terrorism has taken a backseat to other headlines.

Los Angeles is arguably the nation's outdoor-sports capital, home of CicLAvia, an annual marathon of our own (which took place last month), the X Games, professional volleyball and Venice boardwalk skateboarding and basketball exhibitions.

By targeting a marathon, the Boston bomb maker (or makers) took terror in a direction that makes L.A. seem particularly vulnerable: We may never again take safety for granted at open-air events, just as American presidents probably will never again ride in open-top cars.

“This country has done a terrific job at hardened targets — stadiums and arenas,” says Lou Marciani, director of the National Center for Spectator Sports Safety and Security at the University of Southern Mississippi. “I guess the enemy picked a softer target.”

LAPD is responding by increasing security at CicLAvia, which starts Sunday at 10 a.m. at El Pueblo de Los Angeles. While the department can't vet every rider and spectator, it will do one thing that could deter the most violent attackers, police told us: Put undercover cops along the route.

“They provide a very comprehensive security detail for the entirety of the event,” CicLAvia spokesman Robert Gard says.

Experts say that some measures can ensure open-to-the-public events are more secure, but there's no sure-fire way to promise safety besides not holding them at all.

Long-distance outdoor events such as bike rides, parades and marathons present special challenges because there are no fixed entry points, screening, ticketing or background checks. Brian A. Jackson, director of RAND's Safety and Justice Program, believes it's likely that we'll now see screening for spectators at the start and finish lines of marathons, for example.

“These kinds of events are always a security challenge,” Jackson says. “It's not like a stadium, where there are easy places to set up checkpoints and search people for weapons. Looking at the marathon as an example, you couldn't do this along an entire 26-mile route, but you could do it in denser areas of population — places terrorists find most attractive.”

Following reports that the Boston blasts were caused by bombs made from pressure cookers, which physicians said seemed to target the lower extremities of the victims, authorities there searched trash cans near the finish line. There probably will be an increased focus at future outdoor events on securing such receptacles, along with manholes and other urban cubbies.

“Planting bombs in trash cans is a strategy going back to the beginning of the age of modern terrorism in the 1970s,” RAND's Jackson says. “If you put a device there, it will go undisturbed, most likely. And therefore it would be positioned where terrorists want them to be.”

He says some subway systems in Europe have done away with hard trash cans and that others, including Washington, D.C.'s, have reverted to “bomb-resistant” receptacles. That might be an option for L.A., although, Jackson says, “The downside is that it costs quite a bit.”

Marciani of the National Center for Spectator Sports Safety and Security says sealing manholes and doing away with “those types of receptacles that could hold a backpack or IED explosive” is part of any “normal checklist” for a major public event in the streets these days.

Still, a good part of the terrorism game isn't tactical. It's psychological. And denying the aim of the terrorist includes carrying on — going to CicLAvia, the beach, the mall — as usual. Experts note that even with 9/11, fatal attacks on U.S. soil have actually decreased since 1970.

“This has been only one of a few successful attacks domestically since 9/11,” Jackson says. “Terrorism is still a relatively rare event. Our goal should be to figure out common-sense and least-disruptive responses to make the next attack more difficult to plan.”

“We're not going to stop living,” Marciani says. “America's very resilient. I think we'll move on. As a bicyclist or a runner, I wouldn't worry about the next event in L.A. They're doing a good job.”

L.A. elder statesman Steve Soboroff, who in 2011 was vice chairman of the Los Angeles Dodgers, says one of his adult sons was a mile away when terrorists struck Lower Manhattan on 9/11. His grown daughter was four blocks away when the bombs went off in Boston this week. Still, he says, it won't stop him from going out to public sporting events.

“It may affect how organizers put things on, but I don't think it will affect attendance or participation,” he says. “Americans are defiant.”

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