Pedestrian bridge dimensions come down to three figures that interact with each other: how wide the deck is between its parapets, how much space there is underneath, and how much there is above. Each one is easy to state and each one is routinely stated in a way that cannot be built to, which is why so many crossings arrive narrower than the client pictured.
The interaction is the part worth understanding. Clearance underneath sets the rise, the rise sets the ramp length, and the ramp length sets the footprint and a large share of the cost. A client who changes the clearance by half a metre has changed the size of the whole project.
Width means clear width, and it is not the number on the brochure
The useful figure is the unobstructed width between the inside faces of the parapets, measured at the level people actually walk. Overall width includes the structure itself and can exceed the usable figure by several hundred millimetres, so a tender that names one and a supplier who quotes the other will disagree about the same bridge.
As a starting point, a crossing intended for pedestrians alone rarely works below two metres clear, because two people passing with prams or shopping need that much and a single obstruction takes the rest. Two and a half metres accommodates a small maintenance vehicle where one is required. Shared pedestrian and cycle use sits higher again, typically between three and four metres depending on expected flows.
Two details eat into the figure after it is agreed. Handrails mounted inside the parapet line project into the walkway, and lighting columns, signage or drainage upstands placed on the deck reduce the effective width further at the exact points where people tend to stop. Specifying clear width between parapets with a note that fixtures must not encroach settles both.
Clearance underneath is the figure that shapes the project
What the bridge crosses dictates the headroom required beneath it, and this is the number that comes from someone else rather than from the client. A road crossing needs the clearance the road authority requires, which in European practice generally falls somewhere between four and a half and five and a half metres depending on the route and the country, and a margin is usually added for future resurfacing.
A railway crossing is a different order of problem. The infrastructure manager sets the clearance, electrification raises it substantially, and additional requirements follow for protective screening, earthing and the possession windows during which work may be carried out. None of that is negotiable and all of it should be established before a span is priced.
Over a watercourse the governing figure is the design flood level plus a freeboard allowance, which is set by the navigation or drainage authority rather than by the river as you see it. Sites where a boat has to pass beneath add their own headroom requirement on top.
Headroom above the deck
Open decks rarely raise the question, but any structure with an arch, a truss with overhead bracing, a canopy or a roof has to give people room to stand. Two and a half metres is a reasonable working minimum and more is better where cyclists are carried, because a rider sits higher than a walker and a low member over a cycle route is a hazard rather than an inconvenience.
Truss structures deserve a specific note. A through truss with cross-bracing above the deck produces a tunnel effect that affects how safe a crossing feels at night, quite separately from whether the geometry is compliant. Where that matters, an under-slung or a half-through arrangement solves it at design stage.
Parapet height depends on who is crossing
The parapet is a dimension as much as a component. Pedestrian crossings typically call for something around 1.1 metres, while routes carrying cyclists are generally required to be higher, because a rider’s centre of gravity sits well above a walker’s and the consequence of going over the edge is different. National rules set the actual figures, and where a route is shared, the higher requirement governs the whole width.
Infill matters alongside height. A parapet that can be climbed is a different product from one that cannot, and over a railway or a road the requirement is usually for a design that resists both climbing and objects being dropped through it.
How the figures multiply
The chain runs in one direction and it is worth walking through before a site is fixed. Required clearance underneath, plus the depth of the structure itself, gives the deck level. Deck level minus ground level gives the rise. Rise divided by the permitted gradient gives the approach length, and the accessible gradient that applies to those approaches is frequently the reason a crossing does not fit where it was first drawn.
A shallower structure therefore buys ramp length at both ends, which is the practical argument for a deeper engineering conversation about the deck early on rather than treating structural depth as the fabricator’s business.
What the brief should state
Give the clear width between parapets and say that fixtures must not reduce it. Give the required clearance underneath and name the authority that set it. State whether cyclists are carried, because that changes both width and parapet height. State whether a maintenance or emergency vehicle has to reach the deck. Give the headroom above the deck if the structure is anything other than open.
Five lines, and they eliminate the most common source of disagreement after award, which is two parties using the same word for different measurements.
Before the position is fixed
Dimensions on a crossing are not independent choices. They are a sequence, and the first one in the sequence belongs to the road or rail authority rather than to the client. Establishing it before the alignment is agreed prevents the situation where a viable span sits on a site that cannot accommodate the approaches.
Our engineers will run the geometry through from required clearance to ramp footprint at brief stage, and the Fußgängerbrücken aus Aluminium we supply are quoted with clear width and structural depth stated explicitly, so the figures in the offer are the figures that get built.