Today the Department of Little Known San Francisco Curiosities, with the aid of the SFMTA's Moving blog, is here to explain what those yellow circles in the middle of traffic lanes around San Francisco mean.

You know the ones, with the sticks that come out of them pointing different directions, like so.

yellow-dots-sf.jpg

As it happens, these are guides for Muni bus and trolly operators indicating spots where there are breaks in overhead electrified wires. As the SFMTA explains, 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 pointing in different directions are to indicate where the break in power would occur depending on the vehicle being driven — double-length bus vs. streetcar, for example.

Further, to get technical, some vehicles have to power off as they roll through some of these intersections or switches. Per Quora:

The driver must power off [in certain situations]. Firstly, he doesn't want to switch the points. Also, in all overhead wire intersections, at some stage a negative wire must cross a positive wire, and one intersecting wire is protected by isolators — a dead section. An electric bus, under load, drawing 100 amps at 550 volts DC will cause a big, destructive arc if the bus drives onto the dead section (where the wires actually cross) under power. And it's jerky for the passengers, too.

So there you go. Now when you spot one of these yellow "pancakes" waving an arm, you'll be able to sound smart/nerdy explaining this to your friends.

Photo: Quora