Before the bodies cooled Tuesday after a deadly terror attack in Manhattan, conservative commentators raced to proclaim that a good guy with a gun might have stopped the speeding truck that killed eight on a bike path along the Hudson River. This is absurd.

As in Las Vegas one month ago, no good guy carrying a gun would have made a difference in New York on Tuesday. A casual bystander with a pistol would face near-impossible odds in trying to stop a speeding truck. The basic physics of stopping a moving truck with a pistol—or even a rifle or a machine gun—work against even the best-trained and -positioned shooter. Cities can do things to protect themselves against this new and increasingly frequent form of attack. But arming the masses and hoping for a good outcome is madness. Armed amateurs in the middle of terrorist incidents can only increase the carnage.

The basic tactical problem in the Manhattan attack is a variant of one that militaries and police agencies have considered for decades: How to stop a vehicle, such as one carrying a bomb, from getting close to a valuable target and killing people. This is the terror tactic that has blown apart Marine barracks, embassies, and federal buildings. The new variant—prompted in part by anti-terrorism efforts limiting the availability of explosives, and in part by the amateurism of today’s “lone wolf” terrorists—is to use the truck itself as a weapon, driving it through crowds in places like London; Berlin; Barcelona; Nice, France; and of course New York City.

To stop speeding vehicles and prevent attacks like these, security forces use a mixture of physical barriers and weaponry. Look at any major military base or U.S. federal building and you will see these measures: concrete barriers to block all direct access; serpentine pathways into parking areas that make speeding through impossible; heavily armed guards operating from armored booths, with radios to call for help. In the event of an attack, armed police or troops at a checkpoint would fire on a speeding vehicle to stop it.

But this is not an easy shot for even a seasoned marksman. It’s difficult to hit a moving target in a stressful situation like this, even if a shooter has the right weaponry and is firing from a stable, secure position on familiar terrain.

Also, it’s one thing to hit a truck—it’s another matter to hit the parts of a truck that matter. To stop a truck, you have to hit the driver (who sits behind an engine block that can be penetrated only by heavy machine-gun fire or shot through a small windshield aperture); or hit the engine (which can only be disabled by heavy weaponry); or hit the wheels (which are small targets that even when damaged may not stop the vehicle from moving). The battlefields of Iraq and Afghanistan are littered with cases in which U.S., Iraqi, and Afghan forces failed to stop approaching vehicles carrying explosive devices because this is simply very hard to do.

Of course, it would be impractical to place military-style checkpoints at every intersection or vulnerable area of Manhattan. So in any response, armed first responders like the NYPD’s elite counterterrorism squad start from a position of disadvantage because they must respond while moving themselves, instead of from an established checkpoint with concrete barriers to block or slow approaching vehicles.

Now assume you’re talking about a casual bystander walking along the Hudson River who happens to be carrying a pistol. The physics of pistolry make this shot even tougher: Pistols have a much shorter range, approximately 50 meters or so, and fire smaller bullets at far lesser speeds than rifles or machine guns. Pistol fire would not penetrate the engine block and might not even cleanly penetrate the windows or doors of a car because of the physics involved. It would take a one-in-a-million pistol shot that went through the window, seriously wounding or killing the driver, to stop the truck. Unless the Army’s Delta Force happened to be taking a stroll through Tribeca that day while carrying weapons, the odds of a lightning strike are probably higher than a pistol shot successfully taking the driver out.

Marksmanship and physics aside, there’s another huge risk to shooting in a crowded urban area like New York: collateral damage. Tuesday’s truck attack occurred along the bike path of the West Side Highway—a long stretch packed with walkers, joggers, and cyclists on a sunny afternoon. The truck ended its rampage near Stuyvesant High School, which was just letting out, and there were scores of pedestrians including many leaving their offices early to start Halloween. Shooting at the truck would have meant shooting in close proximity to all these people. Many would have likely died from bullets that missed the truck or ricocheted off the truck or the ground in unpredictable ways.

So if it’s impractical to harden every intersection or pedestrian path, and a few good men with guns can’t stop a terror attack, what’s a city to do?

Unfortunately, no city can completely eliminate the risk from every kind of possible terror attack. So long as cities want to thrive, and allow people and goods to move with reasonable amounts of freedom, we must accept that there will be a risk of this kind of attack.

New York, like many other cities, has placed barriers around many of its densest pedestrian areas like Times Square and several parts of Broadway that are closed to vehicles. These barriers have evolved from ugly concrete barriers to less-ugly concrete planters to more artistic features of the urban landscape. These barriers offer the best protection at the lowest cost, but they likely can’t be used everywhere. It would be impractical, for instance, to build concrete walls along every sidewalk in Manhattan.

More police activity—in the form of surveillance, foot patrols, counterterrorism investigations, and information sharing—can help reduce the risk of attacks. But against a lone wolf terrorist like Sayfullo Saipov, these activities have limited effectiveness. Similarly, better rules and systems for keeping explosives or weapons out of terrorist hands can prevent attacks or make them less lethal. But it’s hard to see how any reasonable legal regime would stop a legal U.S. resident like Saipov from renting a truck in a place like Manhattan. And if we could design an immigration system to keep future terrorists out of America—no matter their identity or country of origin—we should do so. Unfortunately, no such perfect system exists, and the Trump administration’s xenophobic approach will do no better at stopping terrorism.

As in Las Vegas four weeks ago, there is little that armed bystanders (or even well-armed police) could have done to stop a speeding truck intent on killing people. The right response came in the arrival of New York police officer Ryan Nash, who fired nine shots at Saipov and disabled him after Saipov’s speeding truck crashed into a bus. Arming bystanders in Manhattan—and hoping they could stop the attack with a lucky shot—could have only killed more people in the crossfire.