From K-QED.

When you come to a pedestrian crosswalk in many parts of the Bay Area and push the button to cross,

you're likely to hear this sound. One big curious listener describes it as a machine gun

sound and wants to know why San Francisco's crosswalks don't use the chirping sound that you might have heard elsewhere.

This week on Bay Curious, we're tuning into the sounds of our urban environment

and learning about their purpose. I'm Olivia Allen Price. of our

sounds of our urban environment and learning about their purpose.

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Wait.

Our listener may think that sounds like a machine gun, but it's officially called the rapid tick. It's part of an accessible pedestrian signal, or APS for short. And that rapid tick is one of a few cues that help blind or visually impaired

people know when the walk sign is on at a crosswalk. We are on the south side of Marcus Street and you can really tell that

because there goes a trolley car. I met up with Frank Welty to get a quick

demonstration. I work as a senior braille and accessible media specialist here at the Lighthouse

for the Blind and Visionally Impaired in San Francisco, California.

We meet outside a Civic Center BART entrance across the street from UN Plaza.

So now I'm walking up to the crosswalk here and you'll hear the locator tone.

That's the slower beeping sound that helps Frank find the crosswalk button.

And I'm positioning myself so that the signal is right here to my right.

And I'm going to push the button.

...