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Have you ever wondered what all those yellow dots were doing in San Francisco streets?

Some have tails, others have a line through them.

These are secret code for Muni’s trolleybus operators. They communicated where the overhead wires join, cross, and most importantly: where and in what direction they split. If the operator needs to change wires, there’s a button they need to press to trigger the switch overhead, and the yellow dots tell them when.

Because the dots mark where the poles at the back of the bus will be when the operator crosses the dot. Because trolley-buses come in both 40 and 60-foot versions, the dots come in a set, 20-feet apart.