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Traffic Bottlenecks

Traffic Bottlenecks > Combating Bottlenecks

Combating Bottlenecks

Minnesota

MN Proposes expanding use of Dynamic Shoulder Lanes (and: "what is a DSL anyway"?)

The Minnesota Department of Transportation presented their plan for a 5-mile long dynamic shoulder lane on I-494 (from Highway 55 to East Fish Lake Road) to the Plymouth (MN) City Council on January 15, 2013. The shoulder would be open to traffic during the peak hours when there would be congestion. It would be closed to traffic otherwise. Overhead green arrows or red X's would indicate the permitted use. The lane would be modeled on the one that currently exists as a MnPass lane on I-35; however, the plans as-presented may allow all traffic — and not just MnPass-eligible traffic — on I-494 during the "open" hours.

So what is a Dynamic Shoulder Lane? In the most basic sense, it is use of the shoulder during peak hours only (hence the term "dynamic" which implies that it is turned on and off, so to speak) as a peak hour congestion mitigation. It may or may not be open to all traffic. Several states have peak hour shoulder use facilities for buses only, including CA, DE, FL, GA, and MD. Other states like MA, VA, HI, and WA allow cars and may or may not allow heavy trucks. (Note: these are not complete lists.) The lanes are closed (i.e., return to shoulders-only) during non-peak hours. In Minnesota, they currently run one on the left side of I-35 between 42nd street and downtown as the continuation of the HOV/HOT lane; that is, it is open to transit vehicles and carpools for free, and for paying MnPass customers. Electronic signs alert drivers whether the DSL is open or closed.

A DSL is not open 24/7 because it is thought that during non-peak times, when traffic is running free-flow more-or-less, then having a shoulder for breakdowns and maneuvers is more desirable. However, during peak periods, when traffic is generally slower, then having another thru lane, if only for qualified vehicles, is more desirable. The use of DSLs was pioneered in Europe and brought to the U.S. in the late 90's. Generally speaking, the reception has been favorable, vis-á-vis the reduction potential of chokepoints. However, there are safety and design concerns that exist too. The FHWA began studies of the issues surrounding DSLs beginning in the mid-2000's and have published several studies and guidance documents thereto.