Perhaps we can agree with George Washington University’s Christopher Leinberger and many others: “walkable urbanism” is a worthy goal. Indeed, let’s simply declare that pedestrian-rich street life, flaneur-friendly walkability, and nice cafés and shops are a key to improving the quality of life in cities and towns.

But there’s just one thing: we must avoid being run over. And unfortunately, in the last few years, we’ve discovered something quite horrible: in the hands of a terrorist, the familiar four-wheeled vehicle can be a frightful weapon.

Nobody has forgotten the April 7 attack in Stockholm, when one Rakhmat Akilov, an immigrant from Uzbekistan, hijacked a brewery truck and drove it onto the sidewalk and then into a store, killing four and injuring 15. According to one report,

The truck mowed down pedestrians along Drottninggatan, a busy pedestrian shopping street. The truck, stolen just blocks away earlier in the day, came to a stop after slamming into the entrance of the Ahlens department store. Photos from the scene showed a billowing cloud of black smoke rising from the store. “I saw hundreds of people running. They ran for their lives” before the truck crashed into the department store, [said] a witness.

In the wake of the carnage, a former Swedish prime minister Carl Bildt wrote on Twitter:

Steal a lorry or a car and then drive it into a crowd. That seems to be the latest terrorist method. Berlin. London. Now Stockholm.

In fact, the dolorous list is longer than that. Perhaps the best known such incident occurred in Nice, France, in July 2016, killing 84. Meanwhile, “ram raiding” has been happening in Israel for years. Indeed, in 2014, an al-Qaeda video celebrated the tactic.

In the bloody-minded eyes of a would-be terrorist, it’s easy to see the appeal of vehicular terrorism: who needs an elaborate conspiracy when one can just get behind the wheel? If the authorities are tracking guns and explosive materials, as well as monitoring travel and airports, why not simply turn on the ignition and start killing? After all, there are plenty of “tools” available; it’s estimated that the U.S. is home to 264 million cars and trucks, and another billion or so vehicles inhabit the rest of the world.

So what to do? How to stop such attacks? Obviously, improvements in homeland security, including immigration vetting—extreme or otherwise—is one answer.

In addition, some will argue that driverless cars are the answer, since they could take away the volitional capability of the motorist to kill. That might all be true in theory, and yet we can point to a few practical problems:

First, according to even the most optimistic scenario, driverless cars are years away—and as we have seen, there are more than a quarter-billion “traditional” vehicles on American roads.

Second, even with driverless technology, do we really expect that the “driver” will have no control of the vehicle whatsoever? Do even the techiest of us think that the computer will control every last movement of every last car, everywhere? And if not—if the driver has any sort of latitude or autonomy—then, in the wrong hands, the potential for mayhem is perpetually present.

Third, speaking of computers and mayhem, we have learned by now—or at least we should have learned—that between Murphy’s Law and malevolent hacking, no cyber-technological solution comes without its own passel of problems, including the problem of deliberate homicide.

So maybe a better way to defend against ram-raiding is, well, defense. That is, a wall, or the equivalent of a wall. As we know, good walls have been making for good neighbors for eons—so why not keep a good thing going?

We can put a wall, or barrier, in the category of “passive defense.” That is, as opposed to “active defense.” Passive defense is just what it sounds like—it’s just there, always on guard. Admittedly, active defense sounds cooler, but by definition, it’s more complicated and thus prone to glitches. And of course, active defense is likely more expensive.

Yet in the meantime, in the wake of the Stockholm attack and all the others, the pressure on public officials and building owners to “do something” will only increase.

Indeed, we might add that here in the litigation-happy United States, there’s an additional prod to take steps. After all, it’s only a matter of time before someone who has been hurt in a vehicular incident, terroristic or accidental, files a lawsuit on the theory that the relevant authority has had plenty of “constructive notice” that such a calamity was coming.

In response to such dangers, this author has no doubt that human creativity and techno-exuberance will produce all sorts of defense mechanisms, including traps, pop-up barriers—and maybe even kinetic projectiles and force fields.

Yet here’s a bet: in the end, when all costs and practicalities are factored in, the most commonly deployed solution will be the humble bollard. That is, those stubby vertical posts that have been used forever to guide traffic and, more recently, to protect buildings.

In fact, we already see bollards, as well as other kinds of barriers, in front of buildings and monuments that we really wish to protect, such as the White House and the Capitol in Washington, DC, as well as other prominent structures, public and private, in Manhattan and elsewhere.

We can’t pull walls around every building, or along every sidewalk, but at least we can put up bollards. Of course, bollards won’t stop every threat, but they will stop a car or truck, and that’s something.

Moreover, bollards, simple as they are today, can be improved. That is, they can be made temporary, or mobile, or self-aware, springing up only when told to do so by a sensor. Thus we can see that bollards could prove to be a hybrid of passive and active.

To be sure, the bollardization of streetscapes, low-tech or high-tech, will not be uncontroversial. By our current reckoning, bollards are ugly and obtrusive. Indeed, we might compare the process of bollard-building to the process of bar-building—that is, the protective bars and grates we often see on domestic windows. To be sure, there’s a tradeoff between aesthetics—and, in some cases, zoning or other kinds of regulation—and safety.

Of course, if bollards prove their value at saving lives, people will likely start to become accustomed to their squat and stalwart visual presence. That’s something we have learned about human nature: if something serves a good purpose, we come to like it, even love it. The little fellas will grow on us!

Still, without a doubt, it’s a shame that it’s come to this. It’s sad that the “sidewalk ballet,” as Jane Jacobs called it, needs new guardrails. But then, of course, even ballet dancers need their barre. As noted, if something is necessary, we soon learn that we can’t live without it.

James P. Pinkerton, a Fox News contributor for 20 years, served as a domestic-policy aide in the White House for Presidents Ronald Reagan and George H.W. Bush.