Soon after the invasion of Iraq in 2003, the street furniture around Westminster transformed into street armour. The Houses of Parliament were regarded as a trophy target for terrorists. An ugly ribbon of steel and concrete sprang up, aimed at preventing a suicide bomber with a truck bringing carnage to the heart of London.

The loser who set out intent on mass murder at Westminster used instead a weapon as commonplace as it is deadly: a car, driven at speed into crowds of tourists and Londoners. They were the softest of targets.

As a city, as a nation and as a world, we must accept that those who seek to trample on our liberal, open and tolerant way of life will turn increasingly to such rudimentary tactics. And we need to move swiftly to protect the crowds who sightsee, stroll and pose for selfies in the tourist heartland of London.

As attacks in Nice, Berlin and now London have demonstrated, cars and trucks are as potentially lethal as bombs and Kalashnikovs — but far easier to procure, and therefore presenting an incalculable risk to the people who congregate in the great cities of the world.

Westminster Bridge is the busiest tourist thoroughfare in Britain, connecting Parliament Square and Westminster Abbey with the London Eye and the other visitor attractions on the South Bank. At any hour during the day hundreds of visitors crowd the pavements and point their cameras, charmed by the miscellany of monuments in the centre of western Europe’s biggest city.

Westminster attack: What we know so far

To mangle Wordsworth’s early 19th-century view of Westminster Bridge, Earth has much to show more fair than the awkward, if endearing, muddle where SE1 meets SW1. Yet in one sense we must turn the clock back to around 1802, a time without motorised transport. Buses, taxis and bikes, of course, must flow across the Thames. But we need to learn from Brussels, which suffered so badly exactly a year earlier.

The main thoroughfare that previously carved from north to south has been closed to normal traffic, and life at the core of the Belgian capital is returning to walking (or cycling) pace.

In pictures: Westminster attack Show all 9 1 /9 In pictures: Westminster attack In pictures: Westminster attack An air ambulance lands after gunfire sounds were heard close to the Palace of Westminster in London PA wire In pictures: Westminster attack MPs wait until the situation is under control in Westminster. 'The alleged assailant was shot by armed police,' David Lidington, leader of the House of Commons, told the house. BBC News In pictures: Westminster attack Crowds gather in Westminster after shooting incident, which police are treating as terror attack BBC News In pictures: Westminster attack Police were also called to an incident on Westminster Bridge nearby AP In pictures: Westminster attack Early reports indicate the car, which mounted the pavement on Westminster Bridge and mowed into around a dozen people, was the same vehicle which then rammed into the railings of the Palace of Westminster, just around the corner Reuters In pictures: Westminster attack Security sources described the suspected assailant as a middle-aged Asian man, who is understood to have left the car before attacking a police officer with a seven-to-eight inch knife PA wire In pictures: Westminster attack Police have asked people to avoid the immediate area to allow emergency services to deal with the ongoing incident AP In pictures: Westminster attack One woman has died and a number of others, including the police officer, have been hurt, according to a junior doctor at St Thomas' Hospital Reuters In pictures: Westminster attack At least three gun shots were heard by those inside Westminster, and proceedings in the House of Commons have been suspended AP

Barcelona, Copenhagen and every other great European city should also take note and begin to protect tourists and locals against mass murderers in secondhand saloons by accepting that heavy traffic and large crowds cannot mix.

“This is a day we planned for but hoped would never happen,” said Mark Rowley, assistant commissioner of the Metropolitan Police, as the toll of human suffering emerged.