Using vehicles to mow down pedestrians, as horrified Londoners witnessed Wednesday, is a terrorist tactic right out of the ISIS playbook. Instead of driving heavy trucks, the terror group’s followers are now using smaller vehicles with similar devastating effect — making it even harder to detect and foil such brutal attacks.

The terror group, which took credit for the London attack, has specifically called on followers to weaponize vehicles and kill “infidels” gathered in outdoor spaces throughout the West.

In November, it instructed such terrorists to drive at “a high speed into a large congregation of kufar (infidel), smashing their bodies with the vehicle’s strong outer frame, while advancing forward – crushing their heads, torsos and limbs under the vehicle’s wheels and chassis – and leaving behind a trail of carnage.”

Sickeningly, the order, which added gruesome detail to a similar 2013 exhortation, encouraged drivers to use “a gun or a knife” to increase “the kill count.”

Allegedly following such orders, ISIS-claimed “soldier” Khalid Masood rented a small Hyundai SUV and ran down tourists walking along London’s Westminster Bridge, killing four and hospitalizing 29, before storming Parliament and stabbing a police officer to death.

Such vehicle-mounted terrorist attacks are effective — and increasingly popular — because they defeat Western security systems, which are designed to screen for bombs and guns. Unlike conventional weapons, motor vehicles are cheap and easy to obtain, and require little training to use.

And there’s no shortage of vulnerable targets. ISIS advises hitting “large outdoor conventions and celebrations, pedestrian-congested streets, outdoor markets, festivals, parades (and) political rallies.” Times Square, the Coney Island boardwalk and the Thanksgiving Day Parade all fall within that target list. (So, for that matter, do candlelight vigils to mourn the victims of such attacks.)

Al-Qaeda has also urged jihadists to plow vehicles into large crowds of innocent bystanders, going so far as to suggest welding steel blades on bumpers to “achieve maximum carnage,” according to a 2010 article in its propaganda rag.

It’s plain from recent attacks that vehicles have become the jihadists’ weapon of choice. The London assault, which was followed the next day by a copycat strike in Brussels foiled by police, was the third deadly car attack in Europe in less than a year. They follow a pattern of similar incidents:

January: An ISIS-inspired Palestinian terrorist driving a large truck jumped a curb and killed four Israeli soldiers; and earlier that month, German police arrested an ISIS-tied Syrian immigrant for plotting to use police cars to ram into New Year’s Eve crowds.

December 2016: A Tunisian immigrant hijacked a truck and killed 12 shoppers in a Berlin Christmas market.

July 2016: An Islamic terrorist in Nice, France, rented a 19-ton cargo truck and plowed into a Bastille Day crowd, killing 86 and injuring 484.

2014: A driver shouting “Allah Akbar!” crashed his car into pedestrians in Nantes, France, killing one and wounding nine.

2014: An ISIS follower in Quebec struck two Canadian soldiers with a car, killing one and injuring the other.

2014: A Palestinian terrorist swerved off the road and slammed into a crowd in Jerusalem, killing an American baby girl and another tourist.

Could it happen here? It already has. In fact, an Iranian-American student may have started the trend in 2006, when he rammed a rented Jeep into a crowd at the University of North Carolina at Chapel Hill, injuring nine. Mohammed Reza Taheri-azar said he was taking revenge for “the deaths of Muslims worldwide.”

Last November, another Muslim student at Ohio State University rammed a car into a crowd and stabbed several people. ISIS called Abdul Razak Ali Artan, who said on Facebook he was protesting “the killing of the Muslims in Burma,” a “soldier.”

What can be done to foil such attacks? Adding barriers between streets and walkways with heavy foot traffic — as they have done in Times Square — for starters. Masood was able to mount the payment along the Westminster Bridge because there are no bollards between the street and sidewalk; and he was able to build speeds in excess of 70 mph because there are no obstacles, such as concrete flower pots, on the sidewalk itself. The large pots, reinforced with steel bars, are commonly used as security barricades.

“Blocker trucks” can also be deployed. After Berlin, NYPD used hundreds of dump trucks to protect pedestrians celebrating New Year’s Eve at Times Square and the Coney boardwalk. Sanitation trucks were also stationed along last year’s Thanksgiving parade.

During last month’s Mardi Gras, New Orleans police deployed portable steel walls that were raised electronically at night to protect Bourbon Street crowds from car attacks.

London authorities, in contrast, reportedly opted for a “lower-key” approach to protecting the tourist area around Parliament in reaction to Nice and Berlin.

Since many of the vehicles used in recent attacks have been rentals, NYPD and other police have been checking with auto rental agencies, especially large truck rental locations, for suspicious renters. Red flags include: SUV or truck rentals, no previous history of rentals, and customers with Arabic surnames, religious head coverings or beards and behaving nervously at the counter.

Reporting to authorities signs of surveillance, casing and targeting by suspicious individuals can also help thwart car attacks. ISIS advises would-be terrorists to survey the route of attack they’ve mapped out — including on the day of attack — for “obstacles, such as posts, signs, barriers, humps, bus stops, dumpsters, etc., which is important for sidewalk-mounted attacks.”

Sperry, former Hoover Institution media fellow, is author of several books on terrorism including the bestseller “Infiltration.”