Wind engineering plays a direct role in pedestrian safety by identifying where wind conditions around buildings reach speeds that can knock people off balance, cause accidents, or make public spaces unusable. When buildings redirect strong winds toward street level, the results range from uncomfortable to genuinely dangerous. Below, we walk through the most important questions about how wind affects people on the ground and what can be done about it.
How does wind around buildings become dangerous for pedestrians?
Wind becomes dangerous for pedestrians when buildings redirect airflow downward or accelerate it through gaps, creating localised gusts that far exceed the surrounding wind speed. A tall building can funnel wind from height to street level, producing conditions that are unpredictable and far stronger than the open-field wind speed would suggest.
Several mechanisms cause this. The most common is the downwash effect, where wind hits a tall facade, cannot pass through, and is forced downward along the building’s face. At street level, this creates sudden, powerful gusts. Another mechanism is the corner acceleration effect, where wind wraps around the edges of a building and speeds up significantly. A third is the Venturi effect, where wind is compressed and accelerated through a narrow passage between two buildings or between a building and a wall.
The risk is not just about absolute wind speed. Gusts that arrive without warning are more dangerous than steady winds of the same speed, because people cannot brace themselves in time. This is why exposed locations near tall, freestanding towers, open bridges, and building passages are consistently the most problematic spots in urban environments. Research from the Rotterdam wind study, for example, identified the Erasmus Bridge and sections of the Willemsbrug as wind hazard locations precisely because of their exposed position above the river, with no surrounding buildings to break the flow.
What are the Lawson criteria and how do they define wind safety?
The Lawson criteria are a set of internationally recognised thresholds used to classify wind conditions at pedestrian level. They define acceptability based on what activity takes place at a location, ranging from sitting outdoors to walking through a busy thoroughfare. A location is assessed as safe or unsafe depending on how often wind speeds exceed defined thresholds for that activity type.
The criteria distinguish between wind comfort and wind safety. Comfort thresholds govern whether a space is pleasant to use. Safety thresholds identify conditions that pose a genuine risk of people losing their balance or being injured. In the Lawson framework, a wind safety threshold is typically set at around 15 m/s, with exceedance probabilities determining whether a location is classified as occasionally risky or unacceptably dangerous.
The Lawson criteria are the standard used for projects in the UK and internationally. In the Netherlands, the equivalent standard is NEN 8100, which uses a similar structure but with its own classification table and Dutch meteorological data as input. Both systems share the same underlying logic: measure how often wind at eye height exceeds a threshold, then compare that frequency against what is acceptable for the intended use of the space. Understanding which standard applies to your project is one of the first questions to settle before commissioning a wind engineering study.
Which building types and designs create the worst wind conditions?
Tall, freestanding towers are the most common source of severe pedestrian wind problems. A building that stands more than twice the height of its surroundings is particularly likely to generate strong downwash and corner acceleration at street level. The taller and more isolated the building, the greater the contrast between wind conditions at height and at ground level.
Beyond height, several design characteristics make things worse:
- Wide facades perpendicular to the dominant wind direction increase the volume of air being deflected downward
- Passages and openings aligned with the prevailing wind create pressure shortcuts that accelerate flow through the gap
- Flat rooftops with no setbacks allow downwash to reach the full height of the facade without interruption
- Large open plazas on the windward side give wind an unobstructed run-up before it hits the building, increasing impact speed
- Significant height differences between adjacent buildings generate turbulence at the transition zone
A useful rule of thumb: if more than 25% of the windward facade of a plaza is open, the risk of wind problems increases significantly. Similarly, street-to-building height ratios below 0.35 mean most wind reaches the street rather than being deflected over the roofline. These proportions matter as much as the absolute height of any single building.
How does a wind engineering assessment identify safety risks?
A wind engineering assessment identifies safety risks by simulating or measuring wind speeds at pedestrian height across a site, then comparing those results against the relevant safety thresholds. The output shows which locations exceed acceptable limits, how often, and under which wind directions.
The two main methods are wind tunnel testing and CFD (Computational Fluid Dynamics) simulation. Wind tunnel testing uses a scaled physical model and is well suited to individual buildings or smaller masterplans. CFD uses computer simulation and is the practical choice for large urban areas, where a physical model would be impossible to build at sufficient scale. We use advanced virtual wind tunnel technology to produce colour-coded maps that make airflow patterns immediately readable, whether for an internal design review or a permit submission.
The process starts with a 3D model of the site, including existing and approved buildings in the surrounding area. Meteorological data provides the wind rose, showing which directions dominate and how strong the wind is across all seasons. The simulation then runs across multiple wind directions, and the results are combined statistically to produce annual exceedance probabilities. These probabilities are mapped against the NEN 8100 or Lawson classification tables to identify where conditions fall into hazard categories. For large-scale studies, such as the city-wide assessment we carried out for Rotterdam, the computational mesh can exceed 500 million cells, with simulations run across 12 wind directions and two seasonal configurations.
What design measures can reduce pedestrian wind hazards?
Pedestrian wind hazards can be reduced through a hierarchy of measures, starting at the urban planning level and working down to individual building details and public space design. Addressing wind early, at the masterplan stage, is far more effective than trying to fix problems after a building is designed.
The most effective measures, in order of impact:
- Urban layout and orientation: Cluster tall buildings so they shelter each other. Keep height differences between adjacent buildings below 30%. Orient streets diagonally or perpendicular to the dominant wind direction rather than parallel, to avoid channelling effects.
- Building volume and setbacks: Use stepbacks on tall facades to interrupt downwash before it reaches street level. A setback needs to be at least 5 metres deep for a building of around 100 metres to be effective. Avoid the widest facade facing the prevailing wind.
- Aerodynamic shaping: Rounded or tapered facades guide wind around the structure rather than forcing it downward. This reduces both downwash and corner acceleration.
- Covered walkways and galleries: Provide direct protection for pedestrians. To be effective, they need a depth of at least 5 metres for a 100-metre building. The area beside the gallery remains unprotected.
- Screens and barriers: Redirect wind locally, but create acceleration at their edges. Useful in specific spots but not a general solution.
- Public space design: Keep terraces, playgrounds, and building entrances away from the windiest locations. A small repositioning is sometimes all that is needed to move from a hazard class to an acceptable one.
Canopies are worth a specific note: they are less effective than setbacks because they simply move the downwash to the canopy’s edge rather than eliminating it. Vegetation can help with comfort but is not a primary safety measure, partly because deciduous trees lose their leaves in winter, exactly when wind speeds are highest.
When is a pedestrian wind safety study legally required?
A pedestrian wind safety study is legally required in the Netherlands when a building project exceeds a defined height threshold or when the municipality includes a wind assessment as a condition of the environmental permit. Under NEN 8100, wind studies are typically required for buildings above 30 metres, though individual municipalities can set their own thresholds and requirements.
In practice, the trigger is often the permit application stage. A municipality may require a wind comfort or wind safety report before approving a building permit, particularly for high-rise developments, large area developments, or projects near sensitive public spaces such as squares, parks, or transport hubs. Some municipalities in the Netherlands have integrated wind assessments into their spatial planning policy, meaning the requirement applies at the zoning plan stage, before individual building permits are even submitted.
For projects in the UK, the Lawson criteria apply and the requirement is typically driven by the planning authority rather than a single national standard. Internationally, requirements vary by jurisdiction. The safest approach is to check with the relevant authority early, rather than discovering a mandatory study is needed when the design is already advanced. A delayed wind report can hold up a permit application by weeks, which in a tight project schedule can have significant consequences for the whole programme. You can find a broader overview of how wind studies fit into the design and permit process on the Actiflow website.
How Actiflow helps with pedestrian wind safety
We specialise in wind engineering assessments for buildings and area developments, with over 21 years of experience working with architects, developers, structural engineers, and municipalities across the Netherlands, Belgium, the UK, and internationally. Whether your project requires a NEN 8100 assessment for a Dutch permit or a Lawson-based study for an international submission, we know the regulatory landscape and can advise you on exactly what is needed.
Here is what working with us looks like in practice:
- We assess your project and advise on which study type and standard applies
- We build a 3D model of your site and run CFD simulations across all relevant wind directions and seasons
- We deliver colour-coded maps and clear visual output that you can present directly to clients, planners, or permit authorities
- We identify wind hazard locations and provide concrete design recommendations to address them
- We work at every scale, from a single high-rise to city-wide masterplan assessments like the one we completed for Rotterdam
- For regular clients, we can start the next day if the schedule demands it
Curious how we can help with pedestrian wind safety? Contact us and we will be happy to discuss your project and help you find the right approach. You can also learn more about us and the work we do.
