There were many items that impacted the decision of whether or not the bridge could/should be widened.

The first thing that needed to be addressed was the existing and forecast 2028 traffic volumes. These volumes were then modeled to see where people were going to/from to determine what type of improvements were needed (i.e. through, left, and right turn lanes).

This information resulted in the traffic forecasts that have 49,000 daily trips at the interchange in 2028, up from 26,000 today. The following three alternatives were developed for the interchange area of 19th St. to 12th St.

ï Bridge widening from four to six lanes (Add a through lane in each direction).
ï Compressed Diamond, complete reconstruction (Add a through lane and left turn lane in each direction).
ï Roundabout (re-stripe existing bridge to add a through lane in each direction).

The Bridge Widening option didnít work because the traffic signal spacing in the interchange area didnít allow for vehicles to get a green at one light and then a green at the next.

This resulted in unnecessary stopping by vehicles reducing the capacity of the roadway. To partially solve this problem the 19th Street signal needed to be removed and access converted into right in/right out.

Additionally, simply widening the bridge was a stop gap measure since this design could not accommodate the forecast traffic volumes and the interchange would need to be reconstructed as a compressed diamond in the future.

Finally, a Minnesota Department of Transportation analysis determined that widening the bridge would be 82 percent of the effort needed to build a new bridge and when this effort passes 70 percent a new bridge should be built.

These reasons lead to the removal of this alternative in August of 2005.

The second and third alternatives, Compress Diamond and Roundabout, were analyzed with 2028 forecast traffic volumes and the following was discovered about the Roundabout Alternative:

ï Better overall level of service than signals, particularly at 12th Street, provided more capacity than signals, provided easier u-turns for business access, accommodated pedestrian movements without impacting intersection coordination and provided a fiscally responsible alterative since the existing bridge could be utilized

So how can a four lane bridge handle as much traffic as an eight lane bridge?

It is because even though the bridges are a different number of total lanes, they both have four through lanes. The four extra lanes on the compressed diamond alternative are there to store left turning traffic while it waits for a green light.

Since the roundabouts donít make traffic wait for a green light there is no need to store traffic in a specific lane.

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Forest Lake Times
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