You'll remember this the next time you're trying to turn left on a busy road, then give up and turn right: A recent study at Penn State found it might be better if we just made most of those left turns ILLEGAL.
Researchers used a bunch of algorithms to figure out the best way to keep traffic moving and prevent accidents. And the simulations found that traffic would move smoother if we BANNED left turns at more intersections.
It depends on the city you're in, and the flow of traffic. But in general, the most efficient setups ban left turns near the middle of metropolitan areas, and only allows them as you get farther away from the city center.