PARIS – Across Europe, the deadly truck attack in lower Manhattan seemed instantly familiar, the latest in a long string of similar vehicle-as-weapon strikes that have left authorities struggling over ways to balance increased security and the traditional free flow of urban life.

As leaders across the Atlantic expressed their sympathies, few could offer concrete recommendations as to how to prevent this increasingly prevalent form of terrorist violence.

In the last two years, many of Europe's major urban centers have been struck by vehicle attacks. In 2016, crowds in Nice and Berlin were targeted in incidents that killed 86 and 12, respectively. In 2017, Stockholm, Barcelona, and London – on separate three occasions – were all struck, in a wave of violence that claimed over 35 lives together. Other isolated incidents have been reported in France.

For the most part, immediate responses to this new form of terrorist violence have focused on erecting physical barriers to better protect pedestrians.

In Paris, city officials announced earlier this year their plans to extend the iconic Eiffel Tower's security perimeter, a proposal that would add public gardens on the popular landmark's eastern and western sides, as well as a "bulletproof fence" on its other two sides.

At the Cannes Film Festival, held each May, sturdy metal barriers as well sturdy planters were installed this year at the entrances to a busy seaside promenade often packed with pedestrians and tourists. The same was true at France's popular Avignon theater festival, where Israeli-made Mobilar vehicles were used to secure entry areas.

In the Nice attack, by far the deadliest of Europe's recent vehicle assaults, a 31-year-old Tunisian national, Mohamed Lahouaiej-Bouhlel, plowed a truck down that city's famed Promenade des Anglais, a similar space where crowds were gathered to celebrate the French National holiday of Bastille Day. That a truck could enter a nominally secured area caused enormous outcry in France, where inquires into police bureaucracy often blamed a complicated system of national and local squadrons that did not coordinate as regularly as possible.

Much the same is true has been true in Britain.

In the wake of the vehicle attacks in London this year, the British capital ramped up its security with the installation of security barriers on its bridges, two of which were targeted earlier this year.

The barricades were installed shortly after the June terrorist attack in the London Bridge area of the city, where three men drove a van into pedestrians before they got out and began stabbing patrons at a nearby market.

The barriers were installed on three bridges: Waterloo, Westminster and Lambeth.

In other cities around Britain, local authorities plan to bolster security at Christmas markets by installing obstructions in pedestrianized areas. Similar measures were rolled out around the country last year following the December 2016 attack in Berlin, where 12 people died in an assault on a popular holiday market.

In Germany, however, the focus has been less on physical barriers than on the question of why a particular assailant, such as the 24-year-old Anis Amri, responsible for the Berlin attack, had not been deported or apprehended.

Amri had been slated to be sent back to his native Tunisia, but his return was delayed for bureaucratic reasons. He had also been under surveillance as a suspected radical Islamist who dabbled in drug dealing. But he had dropped off the radar of intelligence services.

A federal inquiry by a retired prosecutor released last month was scathing in its assessment of law enforcement's handling of the case.

"Gross mistakes were made that should never have happened," said the leader of the inquiry, Bruno Jost.

In the immediate aftermath of the attack, authorities vowed to be more aggressive in deporting those who failed to secure asylum in Germany. But those efforts have been halting, primarily because countries of origin for would-be deportees have resisted taking people back.

The Washington Post's Karla Adam in London contributed to this report.