Tom Vanderbilt in Slate:
Left turns are the bane of traffic engineers. Their idea of utopia runs clockwise. (UPS' routing software famously has drivers turn right whenever possible, to save money and time.) The left-turning vehicle presents not only the aforementioned safety hazard, but a coagulation in the smooth flow of traffic. It's either a car stopped in an active traffic lane, waiting to turn; or, even worse, it's cars in a dedicated left-turn lane that, when traffic is heavy enough, requires its own “dedicated signal phase,” lengthening the delay for through traffic as well as cross traffic. And when traffic volumes really increase, as in the junction of two suburban arterials, multiple left-turn lanes are required, costing even more in space and money.
And, increasingly, because of shifting demographics and “lollipop” development patterns, suburban arterials are where the action is: They represent, according to one report, less than 10 percent of the nation's road mileage, but account for 48 percent of its vehicle-miles traveled.
What can you do when you've tinkered all you can with the traffic signals, added as many left-turn lanes as you can, rerouted as much traffic as you can, in areas that have already been built to a sprawling standard? Welcome to the world of the “unconventional intersection,” where left turns are engineered out of existence. This is not necessarily a new idea: The “Jersey Jughandle” and “Michigan Left” were early iterations of this concept; rolled out widely in the 1960s, both essentially require drivers to first make a right turn, then either looping back or U-turning their way onto the road onto which they had wanted to turn left.