Today's apparent terror attack outside the Houses of Parliament and the use, once again, of a vehicle to inflict harm on members of the public will revive the debate about whether traffic should be banned outside the building.

Security precautions had already been tightened following the attack last year, in which Pc Keith Palmer was stabbed to death and four other people were killed. Khalid Masood had driven a car into pedestrians before attempting to enter Parliament.

The most visible change was the construction of protective barriers on Westminster Bridge, while security at Parliament’s Carriage Gates, where Masood was shot dead, was also enhanced. Most of the other security measures outside Parliament, including the barriers which stopped the vehicle used in today’s incident from getting any further, have been present for many years.

They were installed mainly to stop car or lorry bombs from being driven into the Houses of Parliament. They also aim to make it harder for attackers to use vehicles to breach the building’s defences before mounting an attack on foot.

Problems remain, however. Even though the barriers worked well today, any bomb detonated from a vehicle halted as close as that driven by today’s attacker could have caused significant damage to Parliament itself, as well as devastating loss of life outside.

An even bigger concern is that the increasing use of vehicles as a weapon in terrorist attacks — seen in London at Westminster, London Bridge and Finsbury Park — has brought an increasing threat to the public on the streets outside high-profile locations.

Parliament Crash - In pictures 23 show all Parliament Crash - In pictures 1/23 A Man detained (circled) by Police on Millbank, in central London, after a car crashed into security barriers outside the Houses of Parliament Sky News 2/23 Forensic officerS attend the scene near the Houses of Parliament, Westminster in central London, after a car crashed into security barriers outside the Houses of Parliament Alex Lentati 3/23 Police activity on Millbank, London after a car crashed into security barriers outside the Houses of Parliament Good Morning Britain 4/23 Neil Basu Assistant Commissioner for Counter Terrorism makes a statement paul grover 5/23 Police recover the silver Ford Fiesta involved PA 6/23 Sky News 7/23 PA 8/23 Good Morning Britain 9/23 A forensic officer attends the scene near the Houses of Parliament, Westminster in central London, after a car crashed into security barriers outside the Houses of Parliament PA 10/23 BBC News 11/23 BBC News 12/23 PA 13/23 Reuters 14/23 Sky News 15/23 PA 16/23 PA 17/23 Police activity on Millbank, in central London, after a car crashed into security barriers outside the Houses of Parliament PA 18/23 Police on Westminster Bridge, central London, after a car crashed into security barriers outside the Houses of Parliament PA 19/23 Armed police stand in the street after a car crashed outside the Houses of Parliament in Westminster REUTERS 20/23 Police activity on Millbank, in central London, after a car crashed into security barriers outside the Houses of Parliament PA 21/23 Police activity on Millbank, in central London, after a car crashed into security barriers outside the Houses of Parliament PA 22/23 Police forensics officers work at the scene outside the Houses of Parliament in central London AFP/Getty Images 23/23 Forensic officers by the car that crashed into security barriers outside Parliament PA 1/23 A Man detained (circled) by Police on Millbank, in central London, after a car crashed into security barriers outside the Houses of Parliament Sky News 2/23 Forensic officerS attend the scene near the Houses of Parliament, Westminster in central London, after a car crashed into security barriers outside the Houses of Parliament Alex Lentati 3/23 Police activity on Millbank, London after a car crashed into security barriers outside the Houses of Parliament Good Morning Britain 4/23 Neil Basu Assistant Commissioner for Counter Terrorism makes a statement paul grover 5/23 Police recover the silver Ford Fiesta involved PA 6/23 Sky News 7/23 PA 8/23 Good Morning Britain 9/23 A forensic officer attends the scene near the Houses of Parliament, Westminster in central London, after a car crashed into security barriers outside the Houses of Parliament PA 10/23 BBC News 11/23 BBC News 12/23 PA 13/23 Reuters 14/23 Sky News 15/23 PA 16/23 PA 17/23 Police activity on Millbank, in central London, after a car crashed into security barriers outside the Houses of Parliament PA 18/23 Police on Westminster Bridge, central London, after a car crashed into security barriers outside the Houses of Parliament PA 19/23 Armed police stand in the street after a car crashed outside the Houses of Parliament in Westminster REUTERS 20/23 Police activity on Millbank, in central London, after a car crashed into security barriers outside the Houses of Parliament PA 21/23 Police activity on Millbank, in central London, after a car crashed into security barriers outside the Houses of Parliament PA 22/23 Police forensics officers work at the scene outside the Houses of Parliament in central London AFP/Getty Images 23/23 Forensic officers by the car that crashed into security barriers outside Parliament PA

These factors mean that pedestrianising parts, or indeed all, of Parliament Square would bring security benefits and help police and MI5 as they strive to keep the public safe. It would also make the area more appealing for tourists and those who work nearby.

Balanced against this, of course, would be the challenge of managing traffic flows on alternative routes that would have to be found for the thousands of vehicles that drive past Parliament each day.

Any danger of appearing to bow to terrorism by shutting off the home of democracy would also have to be considered.

But discussion about whether to close the roads will continue.