If you are driving through Mendota Heights you may notice an unusual traffic signal on Highway 149 at
Highway 110. Mn/DOT installed a flashing yellow light for the left turns at the intersection on September 6, 2007. This is one of the first installations of this type of left turn control in the country. FHWA authorized its use (memo) on an interim basis in March 2006. Mn/DOT is studying the effect of the new indication. If it is positive, we’ll be seeing more of these in Minnesota.
What is it?
We are used to seeing the three protected arrows where you can turn freely on the green arrow and 
can’t turn on the red arrow. Then there is the signal with three circles and two arrows. You can turn freely on the green arrow. You can also turn on the green circle, but you have to yield to oncoming traffic and pedestrians (jargon: this is called protected/permitted left turn phasing). The new indication allows traffic engineers to mix and match these operations at different times of day. The new indication adds a second yellow arrow.
In protected mode, it will operate normally – you turn on the green arrow and stop on the red arrow. In protected/permitted mode you can make your left turn on the green arrow and you can also choose to turn on the flashing yellow arrow, but you have to yield to oncoming traffic and pedestrians. Recap for how to drive one of these new signals: Red arrow means stop, solid yellow arrow means clear the intersection, flashing yellow arrow means you can turn left but you have to yield to oncoming traffic, green arrow means you can turn freely.
I’m still confused on the usefulness of this new traffic signal. I still find more usual to use 3 light signal or the five light signal. Unless this type of signal is for experiment. But that’s my opinion. I would like to know more about if I get the chance.