The Michigan system of turnarounds facilitating left turns on divided highways is to speed traffic patterns. Most traffic on divided highways is going to be going straight at intersections, so having to stop traffic in both directions in order for a few people to make left turns in one direction makes very little sense. Instead, the few that have to make left turns are slightly inconvenienced by having to go beyond the intersection and loop back around.

While there is a major road every mile in the Detroit Metro area, relatively few of them are divided in any given area. I routinely am coming north on a 5 lane road, and will need to go east 2 miles. I skip the 4 or 6 lane divided east/west road, and make a right one mile north onto a 2 lane road with a similar speed limit and traffic pattern. I then am able to make a normal left at the traffic light 2 miles west in order to continue going north.

If you're going to take divided roads such that you need to make a left to get on or off them, try to plan your route so that you stay on it for all of your trip in the direction it's headed. The traffic signals are timed preferentially for such roads since they are the main throughfares; barring heavy traffic one should go for many miles without needing to stop for a red light.

However useful Michigan lefts are to car traffic, they are quite a pain to bus and semi traffic. Many intersections allow for buses to make regular lefts at divided highways since their construction makes it impossible to take the turnaround without impeding the rest of the traffic that is turning around.

However, most large trucks would block traffic even more if they had to make lefts at the light, so an additional convenience was added. Most divided highways have a curb with a raised grass shoulder except in the most urbanized areas; across from turnarounds the road for which is only 2 lanes, the pavement tends to extend up onto the curb. This allows for the fact that long trucks are not able to make the turn tight enough to stay on the road and need the extra traction pavement provides when their front goes off the road.

Michigan lefts are just byproduct of the fact that every single person in the Detroit area is expected to have a car, and thus the road system needs to be designed to handle a very heavy load for commuters.