A recent study carried out by traffic engineering researchers at North Carolina State University found that the use of roads without left turns (known as "Michigan left" or "superstreet" design) makes for roads with "significantly faster travel times, and leads to a drastic reduction in automobile collisions and injuries." While the Michigan left has been around since the 1960s, the design is not widely used except in Michigan and to some extent in North Carolina. This study is the first significant examination of the benefits of this kind of intersection.
Drivers, especially those unfamiliar with the concept, may find the Michigan left to be frustrating in practice, because making a left turn in such an intersection often requires a stop, then driving a short distance and having to stop again before being able to join the flow of traffic in the desired direction. However, the wait times are actually less than if the intersection was configured with all the needed delays necessary to implement four-way left turns (which usually stop all other traffic to allow the lefts).
Left turns across oncoming traffic is eliminated with this kind of intersection, which allows the intersection to cycle more quickly, so that more cars can move through it, and less time is spent idling while waiting at a red light. Superstreets are an inconvenience to drivers on the minor road, but are useful where little of the traffic on the minor road is through traffic. Michigan lefts are a better design for true four-way intersections with high-volume roads.
In practice, the lights on the roads that employ this kind of turn are also timed so that flowing traffic on the major street can continue along without stopping. This can mean that, at busy commuting times, taking the surface street can actually be faster than taking the expressway. (In this case, I'm speaking in part from personal experience.)
To be clear, the application of this design is not universal. It would be hugely wasteful to use this kind of design for every intersection. But for high-volume roadways, the delays at the intersection are shortened for all drivers, not just those making left turns.
Like roundabouts, which are often opposed at first because most drivers are unfamiliar with them and initially find them confusing, a slightly more complex design can be more beneficial to traffic flow as well as to safety. Whether this study will make a difference and see the adoption of this practice in other regions remains to be seen.
written by Colette, February 08, 2011
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