Clarence Perry's proposal in 1933 used the "neighborhood unit" to try to encourage prevention of traffic on residential streets. In the 1930s and 1940s, increased highway traffic provoked the publication of the Community Builders Handbook. Written by the Federal Housing Administration, the handbook provided a hierarchy of street types such as arterial, collector and minor access. However, unsafe conditions were still being observed, and street types were not meeting their intent. Following this, cul de sacs and diverters were used in places such as Montclair and Grand Rapids to encourage streets to fulfill their assigned street type. In the 1950s and 1960s, extensive urban development projects concerning housing and highway construction brought increased volumes of traffic and improvements focused on the flow of the motorized vehicle. Colin Buchanan's Traffic in Towns published in 1963 in England introduced his American companions to the concept of "environmental areas", places where pedestrians are safe from motorized vehicles. He acknowledged that there was an environmental capacity to every street, and pedestrian safety, air and noise pollution, and lighting were all of particular concern. This report influenced design in countries such as Sweden and Australia as well as San Francisco's Urban Design Plan. At the national level, the Federal Highway Administration sponsored a study conducted by Donald Appleyard in the 1970s looking at the impacts of traffic on residential streets. From the study (which looked at 500 homes in the San Francisco area in comparison to other international street designs) they found that residents were deterred by the unsafe travel speeds of motorized vehicles. This was a particular concern in areas with child and elderly populations. Noise and pollution decreased the overall livability of the streets, while more social interaction and perceived privacy was found on streets with lower traffic volumes. Stress, occupancy turnover, and adapting to unsafe living conditions were also results of unfavorable traffic conditions. The findings of Appleyard's Street Livability Study inspired the San Francisco City Planning Board to launch the San Francisco Urban Design Plan in 1971 which introduced the idea of the "protected residential area." The goal of the design was to slow down traffic to acceptable speeds in residential areas through infrastructure improvements for active transport as well as traffic calming measures. While the city's focus in street design used to be increasing the flow of motor traffic, this new approach discouraged high travel speeds. Following this plan, San