Already 2017 has seen vehicle-based attacks in Stockholm, London and Barcelona, following the Berlin Christmas market attack and, most lethally, the Bastille Day attack on pedestrians in Nice last July.
In response, leading Italian architect and urban planner Stefano Boeri has suggested: "With a small investment, carefully distributing trees in the most vital and open spaces of our cities, we can temporarily transform our dearest places into clearings and forests."
He suggested that "vessels of different sizes (the standard may be cylindrical, 1m high and 3m in diameter), placed at the entrances of every square or urban public space", could "protect the passage of pedestrians and minimise the risk of mass murder".
Arboricultural Association senior technical officer Simon Richmond describes this as "a great idea". He says: "If there is the appropriate space to place large containerised trees in open public spaces then why wouldn’t we use this opportunity to further help green our cities? It’s a far better use of resources, offering multiple benefits to the population, than trying to establish a safe zone with concrete blocks and barriers."
Trees and Design Action Group coordinator Sue James is more sceptical. "Clearly finding some more attractive, creative approach to defensive measures would enhance people’s experience of an area," she says, but considers that Boeri "likes to use trees more like pot plants", adding: "The problem with trees in 1 x 3m planters is that they are unlikely to grow and deliver the services that he talks about."
Instead, pointing to published guidance by the Government’s Centre for the Protection of National Infrastructure (CPNI), she suggests: "If trees are to be planted, could they be planted in the ground with anti-vehicle surrounds so that there is a permanent solution that is attractive in urban design terms and will improve over time but not need to be replaced as the ‘pot plant’ tree outgrows its pot and dies?"
One of the topics at Ecobuild 2018 Green Blue Infrastructure Seminar sessions next March, which James helps to organise, will be "how we respond to terrorism and public realm safety without re-cluttering our streets", she adds.
Britain has of course had longer to adapt to threat of vehicular attacks given the IRA threat in the 1980s and 1990s, leading in particular to the Ring of Steel around the City of London, combining barriers with a network of CCTV cameras, and for a time, checkpoints. Even here, "the City made some significant improvements to the public realm using nature-based solutions", James adds (see box).
There is now likely to be more of this around London more widely. Among the many recommendations put forward in a review by Lord Harris of London’s preparedness for further terrorist attack, published before the latest wave of vehicle attacks on the Continent, was that "consideration should be given by the GLA [Greater London Authority] and relevant local authorities to the wider installation of protective bollards in areas of vulnerability around London".
The City of London Corporation meanwhile has recently been advised by security experts to install manned checkpoints, rising street bollards and crash-proof barricades around the eastern section of the City.
London Tree Officers Association chairman John Parker says of Boeri’s idea: "I probably hear it suggested a couple of times a year, and there are already examples of planters being used as defences, albeit it usually bedding and shrubs rather than trees, such as outside the Ministry of Justice."
In his own work as tree officer with Transport for London, "I have approved the use of trees in planters in the past, but only when appropriate", he adds.
"Obviously, we like to get trees in wherever possible and appropriate, but there are a few problems which can come with trees in planters. Newly-planted trees in the ground may require watering for two or three years, but trees in planters will need to be irrigated for their lifetime – whether hand-watered or using an irrigation system. Like so many things, the difference between success and failure will generally be down to the quality of maintenance regimes. Public spending restraints may mean this isn’t viable in all situations."
He points out: "Unfortunately, in my experience, planters also attract litter which needs to be cleaned out by someone, usually the local authority, and there can be concerns around sightlines. I know several places where trees in planters have worked very well and several places where they haven’t."
The junction of West Smithfield and Giltspur Street in the City of London was permanently closed in March 1998 as part of the "Ring of Steel" policy. Subsequent landscaping at the western end of the closure incorporated London plane trees and a lawn area as well as circular stone seating and other solid barriers.
Highway engineering firm Haswell Associates’ director John Baines, who designed the scheme, says: "I always try to put trees in, both to deter vehicles and to provide a green lung for the city. Like most people I prefer trees to concrete bollards, and the general consensus now in highways is to soften the streetscape as much as you can. But it depends on the location. There may be a statutory obligation not to obstruct, or visibility issues, or just no room for trees."