Why Do S.F.'s Crosswalks Play a Machine Gun Sound?

The crosswalks in San Francisco, and many other Bay Area towns, play a 'machine gun' sound when the 'walk' sign is illuminated for pedestrians. A Bay Curious listener recalls hearing chirping sounds in other areas. Why do ours sound the way they do? Plus, we get to know an architect who listens to buildings. Additional Reading: An Architect Who Listens To Buildings Reported by Olivia Allen-Price, Annika Cline and Sam Harnett. Bay Curious is made by Olivia Allen-Price, Jessica Placzek, Katie McMurran and Rob Speight. Additional support from Julie Caine, Paul Lancour, Kyana Moghadam, Suzie Racho, Ethan Lindsey and Patricia Yollin.