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Windspeed planning criteria to protect walking & cycling

active travel

30 August 2019

Developers in the Square Mile will have to adhere to stricter guidelines to ensure that new buildings don’t create uncomfortable wind conditions for walking and cycling.

The City of London Corporation’s new guidelines require developers of tall buildings to assess the likely windspeed impact of their proposals. 

Studies are not required for buildings up to 25 metres in height, unless the development lies close to sites such as hospitals or transport hubs, or the site is in an exposed location, such as beside the Thames, or close to an existing tall building.

For buildings of between 25 and 50 metres, computational fluid dynamic (CFD) simulations or wind tunnel studies are needed. Both types of study are necessary for buildings between 50 and 100 metres in height.

For buildings above 100 metres in height, a two-stage analysis is required.

The guidelines set wind comfort criteria for mean and gust equivalent mean (GEM – the peak three second wind speed divided by 1.85) wind speed: 

  • 2.5 metres/second  – acceptable for frequent outdoor sitting such as restaurants/cafes 
  • 4m/s – acceptable for occasional sitting, such as outdoor spaces, balconies, terraces
  • 6m/s – acceptable for entrances, bus stops, covered walkways  
  • 8m/s – acceptable for external pavements
  • +8m/s – uncomfortable for regular pedestrian access

The criteria represent modifications to the so-called Lawson criteria that were developed for the London Legacy Development Corporation and are regarded as the industry standard in the UK. 

The City’s frequent sitting windspeed threshold is lower than the 4m/s used in the Lawson criteria. Lawson also defined 10m/s as acceptable for business walking/cycling.   

The guidelines specifying that mean and GEM wind speed of 15m/s presents a safety risk.

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