Pedestrian Wind Comfort and Safety

Pedestrian Wind Comfort (PWC) is a crucial aspect of urban design and architecture, focusing on ensuring that public spaces are comfortable and safe for pedestrians. High wind speeds can cause discomfort and safety hazards, while calm winds create pleasant outdoor environments. Computational Fluid Dynamics (CFD) software is used to analyze and predict wind conditions around buildings and urban areas to achieve optimal pedestrian wind comfort.

Standards for Pedestrian Wind Comfort

Wind Comfort: The level of comfort experienced by pedestrians due to wind conditions. This can vary based on wind speed, direction, and frequency.

Several standards and guidelines are used globally to assess pedestrian wind comfort. These standards provide criteria for acceptable wind speeds for various activities in outdoor urban spaces.

Four different wind comfort criteria are available on Archiwind, i.e. those by

• Lawson
• Dutch wind nuisance standard NEN 8100.
• Isyumov and Davenport

While more criteria exist, these four criteria were selected in this study because they are considered “complete” criteria, as they address a wide range of activities, including “sitting/standing long”, “sitting short” and “strolling”. The criteria all consist of a threshold value of the wind speed and a maximum allowed exceedance probability of this threshold value.

Dutch Wind Nuisance Standard (NEN 8100) - 2006

In 2006, the Netherlands introduced a standard for wind comfort (NEN 8100) and a practice guideline (NPR 6097) based on extensive research by Verkaik, Willemsen, Wisse, and others. The main goal of this standard was to provide a uniform approach for wind comfort assessment across the country. This aimed to prevent inconsistent wind comfort assessments for the same building or urban configuration from different consultancy companies or institutes.

The standard includes an improved and verified transformation model to provide wind statistics for any location in the Netherlands, though it does not include local building aerodynamic effects, which are considered part of the design-related contribution. A notable feature of the standard is that it allows the user to choose between wind tunnel modeling and Computational Fluid Dynamics (CFD) to obtain this design-related contribution, marking a significant step towards accepting CFD in wind comfort studies. The standard also emphasizes the importance of quality assurance for both CFD and wind tunnel studies.

The Dutch wind nuisance standard includes one wind comfort criterion and one wind danger criterion, divided into different categories based on activities. The comfort criterion is based on a threshold value for the hourly mean wind speed (UTHR = 5 m/s) for all activities. This threshold is relevant for mechanical pedestrian wind comfort and is supported by interviews with shop owners, as discussed by Lawson and Penwarden. Although thermal comfort is important, wind comfort and wind safety typically refer only to the mechanical effects of wind on people.

The exceedance probability (P) of this threshold value determines the “quality class” (A-E) of the local wind climate (see Table 1). The assessment of the local wind climate (“good, moderate, poor”) depends on the activity (traversing, strolling, or sitting).

The safety criterion has a threshold of UTHR = 15 m/s, with a maximum allowed exceedance probability of 0.3%.

Comfort

Safety

Lawson Criteria

Professor T.V. Lawson developed the Lawson Comfort Criteria at the University of Bristol in the 1970s. These criteria are widely used to evaluate pedestrian wind comfort in urban environments. They provide a systematic way to classify wind conditions based on their impact on pedestrian activities.

Versions and Evolution

The Lawson criteria have evolved to incorporate more comprehensive data and improved understanding of wind effects on pedestrians. Although the core principles remain consistent, adjustments have been made to reflect changing urban designs and more accurate measurement techniques.

• Lawson 1978

  The Lawson 1978 wind comfort criteria categorize different levels of wind comfort based on the probability of wind speeds exceeding certain thresholds. These categories help assess and manage the suitability of areas for various pedestrian activities. The criteria are defined using a probability value of 2%.

  Example

  1. Uncomfortable (Category E, Red):
     • Wind Speed: Higher than 7.6 m/s
     • Probability: More than 2% of the time
     • Effect: This is the most undesired category. Areas falling into this category are rare but indicate significant discomfort. If such areas are frequented by pedestrians or cyclists, measures should be taken to improve conditions.
  2. Walking Fast (Category D, Yellow):
     • Wind Speed: Less than 7.6 m/s
     • Probability: Less than 2% of the time
     • Effect: This category is suitable for walking fast but may not be ideal for more stationary activities like dining outdoors. The area meets the requirements for Category D but falls short of the criteria for Categories A, B, and C.

  The thresholds for wind comfort vary depending on local authorities and wind standards. Areas are evaluated based on their designated use:

  • Good Quality: Meets the criteria for the intended use.
  • Acceptable: One category below the intended use.
  • Unacceptable: Two or more categories below the intended use.

  The table below summarizes the wind comfort categories:

  Color	Category	Wind Speed (m/s)	Probability	Suitable for
  Green	A	< 1.8	>98%	Sitting Long
  Blue	B	< 3.6	>98%	Sitting Short
  Light Blue	C	< 5.3	>98%	Strolling
  Yellow	D	< 7.6	>98%	Walking Fast
  Red	E	> 7.6	≥2%	Uncomfortable

• Lawson 2001 ✅

  1. Sitting: Wind speeds up to 4 m/s are generally comfortable for sitting activities, such as outdoor cafes and benches.
  2. Standing: Wind speeds up to 6 m/s are comfortable for standing activities, like waiting at bus stops or socializing.
  3. Walking: Wind speeds up to 8 m/s are comfortable for walking activities, such as strolling or walking to work.
  4. Uncomfortable: Wind speeds above 10 m/s are generally uncomfortable for all outdoor activities.

  Comfort

  Color	Category	Wind Speed	Probability	Effect
  Blue	A	< 4	>95%	Sitting
  Light Blue	B	< 6	>95%	Standing
  Yellow	C	< 8	>95%	Strolling
  orange	D	< 10	>95%	Business Walking
  Red	E	> 10	>5%	Uncomfortable

  Safety

  Color	Category	Wind Speed (m/s)	Probability	Effect
  Orange	F	> 15.0	≥ 0.023%	Unsafe frail
  Red	F	> 20.0	≥ 0.023%	Unsafe all

• Lawson LDDC - City Of London

  This Wind comfort standard is a modified version of the Lawson LDDC criteria referred to as the City Lawson Criteria. The criteria categorize areas based on their suitability for different pedestrian activities, considering the mean and gust equivalent mean (GEM) wind speeds at a 5% exceedance level:

  1. Frequent Sitting
     • Wind Speed: ≤ 2.5 m/s
     • Description: Suitable for frequent outdoor sitting, e.g., restaurant or café seating.
  2. Occasional Sitting
     • Wind Speed: ≤ 4 m/s
     • Description: Suitable for occasional outdoor seating, e.g., general public outdoor spaces, balconies, and terraces intended for occasional use.
  3. Standing
     • Wind Speed: ≤ 6 m/s
     • Description: Suitable for entrances, bus stops, covered walkways, or passageways beneath buildings.
  4. Walking
     • Wind Speed: ≤ 8 m/s
     • Description: Suitable for external pavements and walkways.
  5. Uncomfortable
     • Wind Speed: > 8 m/s
     • Description: Not comfortable for regular pedestrian access.

  Presentations of the results :

  The comfort conditions should be presented using a color-coded diagram using the color coding below. Wind safety results can be overlaid on top of the comfort results, such that any red zone indicates unacceptable or unsafe conditions. Alternatively, a separate plot showing the safety conditions can be provided, in addition to the comfort plot.

  Comfort

  Color	Category	Wind Speed	Probability	Effect
  Grey	A	< 2.5	> 95%	Frequent Sitting
  Blue	B	< 4	> 95%	Occasional Sitting
  Green	C	< 6	> 95%	Standing
  Yellow	D	< 8	> 95%	Walking
  Red	E	> 8	> 5%	Uncomfortable (and/or Unsafe)

  Safety

  Color	Category	Wind Speed (m/s)	Probability	Effect
  Red	F	> 15.0	≥ 0.022%	Unsafe

  • Additional Considerations

    Contextual Adjustments

    • The “Frequent Sitting” category is adjusted to encourage active public spaces with café and restaurant seating.
    • The “Uncomfortable” category, based on previous complaints, includes areas unsuitable for regular public use.
    • Collaboration with City of London planning officers is recommended for sensitive area categorizations.

    Existing and Proposed Developments

    • Existing site conditions and any proposed developments should be tested to ensure no increase in discomfort or safety risks compared to current scenarios.

Isyumov and Davenport Criteria

The comfort criterion by Isyumov and Davenport [7] applies a maximum exceedance probability of “1/week” for the tolerability of wind climate for certain activities. The associated exceedance probability is said to be 1.5% but it is unclear how this value was obtained. 1 hour a week would be 0.6% (1/168x100%), a value also used by Lawson [8] when comparing his comfort criteria with criteria by Davenport. The associated exceedance probability is said to be 1.5%, yet it is slightly unclear how this value was obtained.

Comfort

Safety