Recently, a curious San Francisco resident posted an image of a red button on a stick. This image then went viral on social media as many other residents had no clue what it was used for. You may have seen these dotted around the city in various places. However, it turns out that these have been around for years. Despite being fairly common in the city, you can still easily go years without noticing them.

So, what are these weird buttons and what is their function? The image in question was originally uploaded to Reddit which gained hundreds of replies. Here, the mystery was quickly solved.

It turns out that these buttons are used to control the traffic. As seen, these are situated at a height for bus drivers to access. When pressed, they can turn the lights to red instead of flashing yellow. Often, this is done when buses have to make a sharp turn.

One user wrote: “On occasion, a bus will need to turn right or left (out of the odd middle lane, off of Van Ness). The red button will allow the bus driver to press — then the lights at that intersection will operate in a way that gives the bus the ability to turn without dodging the numerous lanes of traffic. But, since it’s Muni, I suspect the buttons are broken and no longer work.”

While this may not be the most interesting piece of San Francisco history, it’s still something that many people are in the dark about.