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Urban Form, Wind, Comfort, and Sustainability: The San Francisco Experience


In 1985, spurred by the residents' strong interest in the quality of the built environment and in securing comfort in public open spaces, San Francisco became the first city in North America to adopt a downtown plan, supplemented by a planning code, on ground-level wind currents to mitigate the effects of adverse wind. Since then, the plan has mandated that new developments in the downtown and four additional areas in the Rincon Hill, South of Market, Van Ness, and South Beach neighborhoods, all associated with high density or development potential and substantial outdoor activities, be designed or adopt wind-baffling measures so as to not cause ground-level wind current in excess of 7 mph in places for seating and 11 mph in those for walking for no more than ten percent of the time year round, between 7 am and 6 pm, to minimize potential discomfort generated by excessive ground-level wind currents; and 26 mph for no more than one hour per year to secure pedestrian safety.

This research examines whether San Francisco's plan on ground-level wind currents made the city's public open spaces more comfortable and what is the impact on use of sustainable transportation modes. More specifically, it studies (1) whether the plan changed San Francisco's urban form so as to provide a more wind-comfortable environment; (2) whether the wind speed criteria stipulated in the plan effective determinants of outdoor comfort in San Francisco; and (3) whether the plan achieves a wind comfort level that would increase the residents' willingness to use sustainable transportation modes.

Two types of methods were adopted in this research: wind tunnel tests and field studies. The wind tunnel tests, carried out in 2013 at the Center for Environmental Design Research (CEDR), use a boundary layer wind tunnel in which the wind movement in a selected urban area is simulated through use of a scale model of the area's built form. The field study, carried out from July 2012 to December 2012, consisted of pedestrian survey combined with on-site collection of microclimate data, such as wind speed, temperature, relative humidity, and solar radiation. The two methods are effective in addressing the relationships that the sub-research questions seek to examine and the nature of the variables that need to be measured. They also successfully incorporate a mixed-method approach that amalgamates qualitative methods such as observation, interview, and mapping with quantitative statistical analyses.

This research presents the following findings. First, San Francisco's wind planning has changed the city's urban form so as to provide a more wind comfortable environment. Through a series of simulations using the boundary layer wind tunnel and comparing the wind speed ratios at 318 locations in the selected sites of Yerba Buena, Van Ness, Civic Center, and Mission Bay North in the 1985 and 2013 urban form conditions, it was discovered that the overall mean wind speed ratio dropped by 22 percent from 0.279 in 1985 to 0.218 in 2013. It means that the urban forms of the four sites have been changed so that the expected actual ground-level wind speeds have decreased by the same rate. However, there still exist a number of excessively windy places in San Francisco that are associated with specific urban form conditions, including direct exposure of street orientation to the west wind, high-rise building façades that directly meet the ground, and continuous street walls.

Second, through on-site surveys and microclimate measurements, it was discovered that wind speed significantly affects people's perceived outdoor comfort and that 11 mph is an effective criterion that determines outdoor thermal comfort in San Francisco. Significant differences are found in the frequency distributions of people's responses to all of the four comfort measures, which are thermal sensation, wind sensation, wind preference, and overall comfort. Also, the net effects of equivalent wind speed on the comfort measures are strong when the speed is less than 11 mph but become weaker when the speed is 11 mph or higher, meaning that there exists a difference in how much wind determines comfort between the two wind conditions. However, a wide range of dimensions on how people perceive wind and comfort exists, including adaptation, surrender, and avoid, which makes it difficult to judge the effectiveness easily.

Third, the research findings suggest that San Francisco's wind planning does not achieve a wind comfort level that would increase people's willingness to use sustainable transportation modes. It was found that higher wind levels discourage people to wait at transit stop with no shelter, to bike, to walk outside, or to sit outside. Also, significant differences with regard to people's willingness to use sustainable transportation modes exist between when the equivalent wind speed is less than 11 mph and when it is 11 mph or higher. However, the net effects of equivalent wind speed in both wind conditions were not statistically significant, indicating that the criterion does not successfully determine whether people are comfortable enough to be willing to use sustainable transportation modes. Although the criterion was not originally developed to consider the use of sustainable transportation modes, it can be suggested that the criterion can be revised.

A wide range of solutions must be studies for cities in varied climate regions. Cities and regions should not only study and develop their own climate-based ways to make a more climate-responsive city but also vigorously evaluate their effectiveness. Collaboration and cooperation between urban design, urban climatology, and many other relevant fields of expertise is crucial in future research and practice.

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