High left turn volumes in a location with high traffic volume causes congestion.
But believe it or not, there are cities in mainland USA where traffic is stopped to allow left turns do not exist anymore.
Yes, the traffic light with the Green Arrow at the middle of an intersection was eliminated.
No, vehicles still make left turns.  
The only difference is that vehicles make a left turn along with vehicles moving forward!!!
How? With a new and innovative design of an intersection called "Continuous Flow Intersection" or CFI.
http://transportation.ky.gov/Congestion-Toolbox/Pages/Continuous-Flow-Intersection.aspx