Quebec. Jerusalem. Nice. Berlin. Columbus. Stockholm. And now London, where, for a second time this year, a vehicle has plowed into a crowd.

This weekend's attack, which began when three men in a van mowed down pedestrians crossing London Bridge and culminated with a stabbing rampage in nearby Borough Market, has resulted in seven fatalities, not including the perpetrators who were all shot and killed by police, and 48 injuries. As this most recent attack reminds us, “rammings” have become mainstream—and the trend is worrisome.

After authorities made it much more difficult to hijack planes and obtain weapons of mass destruction following 9/11—depriving terrorists of the means to launch spectacular attacks—extremists shifted to simple, easy-to-execute acts of violence like mass shootings and automobile rammings. The unsophisticated and omnipresent threat posed by vehicular terrorism is now forcing those entrusted with security to rethink their paradigms.

Foolproof solutions to instances of low-tech terrorism remain elusive for a variety reasons.

Following the attack in Nice that killed 86 people in July 2016, the Islamic State published a guide for would-be attackers, noting that vehicles are “extremely easy to acquire” and unlikely to arouse the suspicions of citizens or authorities. The group wasn't the first to suggest such an approach. Al-Qaeda championed ramming as an effective method of terrorism years before ISIS came along. In a 2010 article titled “The Ultimate Mowing Machine” in Inspire magazine, a propaganda publication of al-Qaeda in the Arabian Peninsula, the author gave guidance to terrorists on how to choose targets, outlining which vehicles would likely result in the highest casualties. For such attacks, the barrier to entry is remarkably low; the main skill required is the ability to drive. Indeed, as a recent Transportation Security Administration report warns: “No community, large or small, rural or urban, is immune to attacks of this kind.”

Foolproof solutions to such instances of low-tech terrorism remain elusive for a variety reasons, ranging from political reluctance to clamping down on civil rights and liberties, to the cost-prohibitive nature of securing major public spaces. In particular, pedestrians, especially those congregated in large crowds for special events, are incredibly vulnerable to attack.

Whether a ramming succeeds or fails in killing scores of innocent people comes down to four critical elements. This four-point framework is key to understanding vehicular terrorism and safeguarding against it.

Density

How tightly packed are potential victims? Think open-air markets, parades, concerts, or protest marches. The more people there are in close proximity to each other, the more enticing the target for terrorists.

Confinement

How hemmed in are potential victims? It's not enough to strike a large, jam-packed group. Violent extremists also want groups that are trapped, with nowhere to escape once an attack commences.

Access

How approachable is the target population? Crowds inside a stadium are dense and confined, but likely protected from being run over by a truck. Terrorists require unobstructed vehicle ingression—a direct path to the people.

Mass

How big is the vehicle? If there is one lesson we can learn from recent attacks, it's that the larger and heavier the means of ramming, the greater the fatality toll. Put simply: Trucks and buses are deadlier than vans and cars.

Domestic audiences in Western nations understand that even spending billions on counterterrorism can never guarantee the prevention of each and every attack. So what options are available to authorities determined to protect citizens from vehicular attacks?

For governments, complete protection is neither possible nor feasible, but proactive efforts can mitigate threats. Some conventional steps have proven effective. For instance, “beat cop”-style patrols by local law enforcement and a visible public safety presence in open spaces can deter would-be attackers from going after certain targets. For planned events in specific locations, counter-surveillance is also useful. Maintaining detailed records (especially photos) of suspicious activities that can be shared by authorities—and, when appropriate, disseminated to the public—can be instrumental to identifying and locating persons of interest to law enforcement.

Nothing will be more useful than denying vehicular terrorists access to high-impact locations.

But nothing will be more useful than applying the four-point model and denying access to high-impact locations. Defending the public begins with a simple question: What will it take to prevent a truck or bus from reaching a cramped and constricted gathering? The answer will require authorities to identify all access routes and to place formidable obstacles in those paths (for example, erecting concrete or steel jersey barriers and stanchions as well as employing oversize vehicles like buses and sanitation trucks as blockades).

Of course, one of the most significant lessons of the post-9/11 era is that public safety must involve the public. “See something, say something” is more than a catchy slogan. It's a life-saving best practice. Ensuring that the population at large knows what to look for and who to call if suspicious activities are spotted remains a critical step in the right direction.

Finally, there is one simple thing everyone can do to help foil vehicular terrorism: Secure their vehicles. Perpetrators of political violence understand that the bigger the means of transportation, the greater the damage. Yet, most terrorists do not have access to lorries and buses, which is why several of the deadliest incidents, like the attacks in Jerusalem and Stockholm, have involved stolen commercial vehicles. It might sound trite, but never leaving keys in unattended trucks is a common sense measure that can go a long way in safeguarding the masses. Terrorism—especially jihadist terrorism—is often an opportunity crime.

Getting authorities, businesses, and the general public to recognize the growing danger of vehicular terrorism is an important step toward reducing its impact. And the first step begins with awareness. After all, a vigilant society is a safer society.

Colin P. Clarke is a political scientist at the RAND Corporation and an associate fellow at the International Center for Counter Terrorism. He is the author of Terrorism, Inc.: The Financing of Terrorism, Insurgency, and Irregular Warfare. Louis Klarevas is associate lecturer in Global Affairs at the University of Massachusetts—Boston and the author of Rampage Nation: Securing America from Mass Shootings.

This commentary originally appeared on TheAtlantic.com on June 4, 2017.

Commentary gives RAND researchers a platform to convey insights based on their professional expertise and often on their peer-reviewed research and analysis.