The social power of infrastructure design


Using a variety of examples from across the UK and Europe, Martin Knight advocates for connected infrastructure, creative reuse and innovation in the use of materials to encourage behavioural change to more sustainable and active modes of transport. In this, he draws out the social value and benefit that can be created by sensitive infrastructure design, in particular, bridge design in which his practice specialises.

About the Author

Martin is one of the leading UK architects specialising in the design of bridges and transport corridor infrastructure. His award-winning practice, Knight Architects, champions infrastructure design that creates social value and addresses the Climate and Biodiversity Emergency. Having completed more than 50 bridges around the world, their approach to design has been hailed as exemplary by bodies including the UK National Infrastructure Commission and the Design Council in the UK.

Contact:  or Knight Architects +44 (0) 20 3978 1555


As an architect working in the field of bridge design – the acknowledged territory of engineers – I am aware of the focus on function, from which form is supposed to follow, at least in modernist design philosophy. This order encourages efficient use of materials and provides solutions that are appropriate to their span and the load carried. In turn these solutions are more likely to fit with their context, or at least to not stand out unintentionally. But what exactly is the function of a bridge?

Ulm Kienlesbergbruecke, DE. Photographer: Wilfried Dechau

In my opinion, the highest function of a bridge is to make people’s lives better.  This is a much broader and deeper challenge than simply carrying a load safely across an obstacle, which is arguably the lowest measure of performance. A truly successful bridge should also look beautiful; its design should respect and even enhance its place. Afer all, for centuries great cities and communities have been founded upon great bridges – the design of civil infrastructure is a measure of how a culture values itself and its people – and philosophy and architectural theory has used the bridge as a central motif since Heidegger’s Bauen, Wohnen, Denken lecture in 1951. This can be simplified as the place first defines the bridge and thereafter the bridge defines the place. History shows us that a useful bridge should be positioned and aligned to provide the most attractive route to users and, in so doing, it should not make the situation worse for others. It should act as a catalyst for improvement to the surrounding public realm. It must provide socially valuaable connectivity – for example by linking with jobs, education, amenities and nature.

In the age of a Climate Emergency and at a time of energy crisis, the function of a bridge should now include encouraging modal shift towards energy-efficient forms of mobility, such as rail, tram and active travel modes. Even while the use of electric cars increases it is possible, or even likely, that modal shift will accelerate away from private car use in cities where problems of congestion are unchanged by the power source. These problems remain even when the car is parked (which is most of the time) and recent advertising campaigns have cleverly illustrated how space-hungry private cars are compared to cycling and walking.

Once a user is taken from the comfortable, isolating cocoon of their vehicle, they become much more aware of the surrounding environment – and other people – and it is this immediacy which demands design that is safe, enjoyable, inclusive, durable and beautiful. For a bridge designer, the sensitive needs of pedestrians and cyclists are the greatest and most rewarding challenge. My professional interest increasingly lies in the social value and benefit created by bridge design, as much as their architectural contribution to cities and landscape. And when good social intentions and beautiful design combine, the results are wonderful and long-lasting. Yet supremely functional.

Whether we like it or not, our world is changing. And, as designers of buildings and structures, we are both causing and reacting to change. Of course, for many of us, it is the environmental change to the global climate – now more commonly referred to as the Climate and Biodiversity Emergency – which occupies our thoughts. This is accompanied by, and often intimately linked with, social, cultural and economic change. The world never stands still and so we must move with it.

As designers, our actions must be driven by social purpose, and this requires a comprehensive understanding of a project’s intention and who it will benefit even before considering what the design solution may be. But we are small players on a big stage. Modern society relies upon cities as the focus for human dwelling. Far beyond the reach of individual controlling figures, it is the aggregate total of our efforts as designers, as planners and architects and engineers and landscape architects, which shapes this change and will deliver the purpose of our clients. It is clear we have an overarching duty to society to ensure the sum of our work is beneficial and this means taking individual accountability for the total result. This is an interesting challenge as it requires a faith that others will do the same and an optimism that, if we all do our best, future generations will enjoy the reward of our labours.

For designers, this duty to society demands thinking beyond the footprint of a building, to consider how the surrounding environment is changed by the building. For designers of infrastructure, it requires extraordinary imagination far beyond the base requirements of moving people or materials from A to B. Together, it is the positive interactions of people, spaces and places which create successful cities. This has been recognised by many, from Jane Jacobs to Jan Gehl, who stated “Only architecture that considers human scale and interaction is successful architecture”, culminating in his famous call to consider: “First life, then spaces, then buildings – the other way around never works”.

Looking to building in the future, it is helpful to consider what societal needs may be and what we can do now to fulfil these needs. Arguably the most urgent need is to continue to provide access to mobility without a heavy carbon cost. There are numerous ways to achieve this. First, by applying a strict carbon hierarchy to the assessment of a project at its outset, it should be possible to rule out the building of structures that are unnecessary.

The construction of structures that are not truly needed is not as extraordinary as it sounds, as there are numerous examples of bridges intended to satisfy a political need rather than an actual need, where an equivalent functionality could be achieved with little or no carbon cost. Contemporary examples include by changing the provision for vehicles and reallocating existing highway space to active travel uses. This is simultaneously the easiest physical change and the most difficult political change, imposing behavioural change upon society at large, and in doing so causing short term and indiscriminate inconvenience. This approach is powerful medicine and should be implemented with careful anticipation of the consequences, for example by increasing the provision of public transport (including beyond the typical commuter hours for workers on early and late shifts) and ticketing strategies to compensate those who cannot rely upon private car use. The radical temporary changes seen in Paris, London and other cities around the world during the corona pandemic, many of which remain in permanent form, demonstrate this approach is valid with clear leadership. And it is by far the most efficient in terms of both embodied carbon and carbon in use, with little or no new construction yet significant modal shift to zero-carbon mobility.

The decision to provide new infrastructure does not necessarily mean building anew and there are increasingly numerous examples of existing infrastructure being rehabilitated and repurposed. The most well-known example is the High Line in New York but every city has examples of smaller, discrete bridges whose refurbishment could provide significant new value without the heavy investment in carbon of a new build. If only their potential could be identified early enough, and designers were encouraged to think of them as a creative catalyst rather than a liability. The world of infrastructure lags behind the world of buildings in thinking innovatively about re-use. London has recently seen a dramatic increase in the retention and extension of buildings, often exploiting excessive redundancy in their original structure with intelligent analysis. This approach can achieve much greater load-carrying performance in combination with new lightweight components, and this trend seems likely to accelerate due to financial incentives based on carbon performance.

HyLo Building, London, UK.

HyLo Building, London, UK. Split-image. Courtesy AKTII / HCLA

The award-winning HyLo building in the City of London, by engineers AKT II with HCL architects, retained the existing 16-storey tower and extended it upwards by 70% and sideways by 24%, more than doubling the nett lettable space while saving 35% of the carbon of a new-build scheme.

Ever since the understanding of material properties allowed engineers to design structural forms that span further than individual components, the importance of achieving lightweight solutions has been evident in the best examples of bridge design, at whatever scale. This is elegantly illustrated at the Firth of Forth in Scotland, where neighbouring cantilever, suspension and cable-stayed bridges demonstrate the state of the art of 19th, 20th and 21st century engineering. At the smaller end of the bridge design scale there are opportunities to exploit materials with light weight and high strength, or low carbon content. These provide new build solutions which improve connectivity with an emphasis on minimum carbon cost.

Flow Footbridge, UK. Courtesy Network Rail / Knight Architects

Knight Architects’ design of the modular ‘FLOW’ footbridge system for Network Rail will replace numerous remote, rural level crossings in the UK by combining innovative design with intelligent use of carbon, natural and glass fibres. The prototype achieved a 40% reduction in cost in a structure that is half the weight of the equivalent standard steel bridge. Reducing superstructure weight allows further savings in the foundations – which are built without concrete – as well as shortening programme and reducing the size of construction equipment.

As the railways expanded in Europe through the 19th and 20th centuries, a vast number of structures were built that later became redundant as the network declined. These are often suited to repurposing, by combining robust existing foundations and support piers with new, lightweight spans.

Alloa Swing Bridge, Scotland UK. Courtesy Knight Architects

Knight Architects with S82 Consulting has proposed the re-imagining of the support piers of the 1885 Alloa Swing Bridge to carry a replacement deck for pedestrians and cyclists, more than fifty years after the railway and bridge deck were dismantled. In doing so, the carbon invested in the original construction can provide further returns for society. The natural obstacle of the wide river Forth currently means local amenities must be accessed by car, but a replacement deck would link people to greater work and education opportunities with zero-carbon travel.

In certain circumstances, we may even see a departure from absolute light weight solutions towards those which provide robust structures and embrace the carbon storage potential of timber. An extreme example is the 2010 competition-winning design for Margaretengürtelbrücke in Vienna, by structural engineer knippershelbig and Knight Architects, which was planned to link two urban parkland areas across a busy traffic corridor. The linear form of the bridge design evolved from the landscape at each end and its ‘massive minimalism’ was conceived in counterpoint to the rich architectural language of the neighbourhood, including the historic 1899 Margaretengürtel Station building by Otto Wagner.

Margaretenguertelbruecke, Vienna AT. Courtesy: knippershelbig / Knight Architects

The design of the hybrid structure proposed a combination of a glulam core of Spruce lamella and a Larch outer layer to provide durability and protection against moisture. The bending radii of the glulam layers are achievable using standard production techniques and areas of peak stresses reinforced with threaded stainless-steel rods. The robust construction method is combined with the concept of a semi-integral bridge to reduce maintenance costs. Although unbuilt, several examples of this structural concept have been subsequently completed by knippershelbig, including a family of spans for the Remstal Garden Show in May 2019.

The innovative development of traditional timber building techniques is a graphic demonstration of elegant and environmentally responsible construction, being both a carbon sink and an efficient use of resources. Using a locally common building material with a low processing cost, it was estimated the Margaretengürtelbrücke superstructure would contain approximately 795t of stored CO2 within the 960m3 of wood and would consume a fraction of the energy in construction of a concrete or steel bridge of the same span.

Returning to the highest function of bridges – to improve people’s lives – our future bridges must prioritise the needs of the weakest in society, to provide safe, enjoyable routes, spaces and places and to improve accessibility and social inclusion. This can be a major challenge for bridge designers, particularly in a world where topography and the conflicting requirements of existing infrastructure force bridges up and over, resulting in excessive gradients or distances. Unpicking the existing routes to provide new links in urban settings where land ownership and building patterns appear to dictate solutions will not be easy, or cheap, but it is essential.

The Bowline, Scotland UK. Courtesy: Fraser Rankine Landscape Architects

Inspiring examples already exist, including the award-winning Bowling Harbour and The Bowline by Rankin Fraser Landscape Architecture for Scottish Canals. This combines the creative re-use of heritage infrastructure to provide an accessible and enjoyable new community asset, one that benefits nature and biodiversity as well as delivering social value for people through improved health and wellbeing and benefits to the local economy and arts.

This kind of creative re-use both accommodates change caused by aging infrastructure and helps to accelerate change towards a less car-dependant, low carbon urban transport system. Indeed, using the existing annual health inspections of all structures, city authorities could easily predict where and how these changes can be made so that, rather than tearing them down and rebuilding when they can no longer sustain heavy traffic loading, these structures can be repurposed for pedestrian and cycle use within a changed network. The Highline in New York stood redundant for decades, protected by a dedicated band of supporters, before its fantastic potential was realised. It is now a model of creative re-use and illustrates how new networks can be formed with minimum cost compared to the increasing cost of upkeep.

While looking to the changing future we have also to deal with the immediate consequences of the Climate Emergency. Extreme weather events, whose frequency used to be measured in hundreds or thousands of years are now measured in decades or even years, so we must ensure what we design now is ever more robust and reliable. There are many ways of achieving this resilience within and beyond the structures we design, including by recovering highway space for trees, which provide shade and habitat as well as consuming CO2, to providing river and coastal flood protection schemes. Opportunities must be imagined where the benefits are multiplied. This is well illustrated by the Knostrop Bridge (Knight Architects with Mott MacDonald, Arup) which was completed in 2017 as part of the Leeds Flood Alleviation Scheme. By providing a replacement crossing of the River Aire, using a new weir structure to provide supports for a series of lightweight steel spans, a section of the national Trans Pennine Trail was made accessible and a modest new landmark added for the daily enjoyment of recreational and commuter users alike.

Knostrop Weir Bridge, Leeds UK. Courtesy: Knight Architects/Mott MacDoinald

Knostrop Weir Bridge, Leeds UK. Courtesy: Knight Architects/Mott MacDonald.

This approach to multiplying the benefits of a project is unlikely to be realised by conventional, linear thinking. Collaboration between architects and engineers encourages creative dialogue that can result in innovative solutions, which combine form and function to address multiple challenges and solve problems far beyond their original parameters. In doing so, much greater value is achieved for the financial and carbon investment.

Dafne Schippers Bridge, Utrecht, NE.

Dafne Schippers Bridge, Utrech NE. Courtsey: NEXT Architects/Arup.

The Dafne Schippers Bridge (NEXT Architects with Arup) in Utrecht integrates a lengthy approach ramp into the landscape surrounding an infant school, creating a unique sense of place and a strong identity of connection for the city of Utrecht. The fact that cycling is at the heart of the solution is arguably natural to people in the Netherlands, but not to the rest of us. Or at least, not yet.

Many of these examples have ingredients that are important to cycle users and especially to the huge number of people who are “cyclists-to-be” who may find cycling simultaneously attractive and intimidating. Perhaps the most powerful factor in securing the commitment of this important group is providing physical separation from highway modes with some degree of segregation from pedestrians. This means cyclists are not sharing the road with motor vehicles and this allows these two slow modes to co-exist safely. This approach has been referred to in Denmark as “forgiving infrastructure” with comfortable widths and gradients that allow novices to co-exist with experienced users, and which accommodate the speed differential created by some types of modern e-bikes and scooters. The sensitivity of users means that design of cycle infrastructure should not be thought of as a type of highway design. Rather it is a more complex, idiosyncratic and chaotic activity where rules flex and change. Typically human, in other words! Identifying that cycling is a social activity, so space must be allowed for cyclists to ride side-by-side, is just one of the brilliant observations in the Design Manual for Cycle Traffic, published by the Dutch CROW Fietsberaad.

When new developments and road infrastructure are planned, opportunities must be taken to include dedicated Active Travel provision away from the highway edge, so that new residents are not dependent upon private car use from the beginning. The East Leeds Orbital Route in the UK is a good example where cycle and equestrian routes were planned within a linear landscape park, with frequent crossings of the main highway, thereby avoiding the segregation that infrastructure can itself cause. The phrase “coming from the wrong side of the tracks” illustrates the division we can inadvertently create while trying to connect. This remains true for flora and fauna, just as for people, and is being addressed in the increasing popularity of ‘green bridges’ which seek to provide continuity of habitat and re-establish routes severed by linear infrastructure.

Building on the idea of “forgiving infrastructure”, we should be aiming for “generous infrastructure” that not only facilitates energy efficient, healthy and socially valuable A to B journeys but is regenerative in its aims. It could support thriving flora and fauna, sustainable drainage systems, provide trees for shade and undergrowth for habitat, as well providing common places for people to meet and dwell. In other words, in everything we design, how much bigger can our thinking be?

Many of these ideas are familiar and proven because they were in place before the rise of the private motor car, but returning to them means reversing the decades-long growth of car traffic in cities. As we saw during the corona pandemic, beneficial change requires strong leadership. As designers we can support our politicians and clients to create positive briefs and encourage the sharing, celebration and reward of the best examples. When it comes to creating sustainable urban environments, now and for the future, it is not enough only to be good designers – to enjoy our jobs in quiet luxury – we must be advocates, ambassadors and agitators for change and for good design in all areas of life, including bridges.

There will be resistance to give up what we are used to, so it must be made easier to change than to not. This can combine elements of both the carrot and the stick, for example by providing segregated cycle lanes away from traffic and through imposing penalties for private cars, although this must be part of a comprehensive strategy which ensures generous public transport so as not to adversely affect poorer people. As highway traffic reduces, road space can be increasingly returned to pedestrian use, creating an attractive flywheel effect so that parts of cities – such as the Strand, Aldwych in London – thought long lost to pedestrians can be reimagined.

The Strand Aldwych, London, UK. Courtesy LDA Design / Transport for London

What seems or impossible or hard at first will become easier as the benefits accrue, until it is the norm, and society, politicians and asset owners all embrace the opportunity to change.


This article was first published in Bautechnik magazine, August 2023.