Muni’s mysterious yellow dots on the ground finally explained

You’ve seen the yellow dots on the pavement all over San Francisco and wondered what they are, so the SFMTA blog finally offered a pretty interesting explanation! From the SFMTA blog:

Among Muni staff, these modified circles tend to go by nicknames — tadpoles, frying pans, hamburgers and pancakes…They help operators time their acceleration properly as their electric trolley poles and train pantographs pass through the “breakers” that connect different sections of wire. The arms and gaps on the circles indicate which vehicles they apply to, based on the vehicle type (short or long trolley bus) and the direction of approach.

Commenter Robert Parks, geeking out over the transit trivia, offered a more in-depth explanation of why some of the dots look split while others are filled:

In the picture: you have filled dots with a tail (pancake), and a split dot with a tail (hamburger).

These are variations on the basic marks. A filled circle with no tail indicates that the breaker (the insulator in the overhead) belongs to the 40′ trolley that is approaching the dot in a straight direction.

 

 

A split dot indicates that the breaker belongs to a 60′ trolley artic, also moving in the straight direction in the lane.

The tail points towards the trolley crossing the lane at an angle…in this case the trolley turning from southbound 11th to outbound Mission.

Since the marks for the turning 40′ trolley are doubled, the operator would know that it is a long breaker, not just a short crossing or isolator.

 

 

Out of view below the foreground is the doubled turning breaker for the 60′ trolley.

An unfilled circle is the target point for a 40′ trolley to trigger an overhead switch using the inductive coil (which is activated by pressing the turn signal). An unfilled circle with “TA” in front of it means it is the target point for a 60′ trolley.

Mystery solved!

Photo by SFMTA Blog. Featured photo by Muni Doug.