Two minutes of green time sounds like a lot, but at some major intersections, that is how much time there is to be broken up between the different traffic and pedestrian movements.
Assuming an even split in traffic, half of the green time goes to the east/west traffic, half goes to the north/south traffic.
Yellow time must be provided to notify traffic that the green signal will be ending.
Additional all-red time is added to prevent crashes between traffic starting up and vehicles going through the intersection late in the yellow phase.
Adding a turn arrow takes away not only the time for the arrow, but also the time for the yellow and all-red phases that go with the arrow. You are now down to only 30 seconds of green for your approach, enough time to move about 12 cars per lane.
Pedestrian movements must also be accounted for when considering signal timings. The Walk interval gets people started across the street, the Flashing Don't Walk time allows enough time for a person who just steps into the street to get all the way across at standard walking speed.
