Turn Lane Case Study

Development Type: Farms of Grant Residential Development
Services Provided: Access Evaluation
Client: Redstone Builders

Opening a new access onto a public road can be tricky. Public agencies are increasingly examining the number of access points and how those accesses will impact traffic operations. And rightfully so as every access creates new conflict points and reduces safety, with some having greater impacts than others. A proper review can determine the extent of the impacts and whether mitigation is needed to provide for safe and efficient traffic operations, be it turn lanes, traffic control, or limited access options.

The Farms of Grant is a proposed residential development located in the City of Grant, Minnesota. A new road to provide access to the 30-some homes would create two new intersections with County Roads. Given the expected trip generation, the capacity of these intersections was not in doubt. Instead, the primary concern was the safety of the intersections and, more specifically, if turn lanes on the mainline should be provided to improve safety.

It’s undeniable that turn lanes improve safety. The question becomes the magnitude of that improvement and whether the cost of the turn lane is worth the benefit it would provide. Our contract was to provide the evaluation identifying the need, if any, for turn lanes.

To evaluate the need for turn lanes, we used the Minnesota Department of Transportation’s (MnDOT’s) guidance from their access management manual. This document presents the following nine warrants to indicate the need for turn lanes:

– Presence of a Passing Lane/Climbing Lane
– Limited Sight Distance
– Nearby Railroad Crossing
– Traffic Signal Control
– High Volume of Heavy Truck Traffic
– School Access
– History of Crashes Correctable by Turn Lanes
– Traffic Volumes above Certain Thresholds

Chapter 3 of the full document with this criterion is located here (pages 46 and 47) if you want to examine the full criteria. Based on this, we recommended a right turn lane for the mainline of one intersection along with a strengthened shoulder (de facto right turn lane) and right-of-way for a future by-pass lane for mainline of the other intersection.

To their credit, Washington County, the reviewing agency, has also been considering the appropriate warrants for turn lanes and examining their overall effectiveness. Although not yet published, the County’s experience has indicated that by-pass lanes are not effective compared to left turn lanes and that both left and right turn lanes should be provided when the mainline volume reaches approximately right turn lanes on the mainline County Roads.

With a difference in recommendation between us and the County’s traffic engineers, we had a couple meetings to further discuss the issue. Other factors we collectively considered included:

– Potential for additional houses in the development
– Split of traffic between the two intersections (i.e. how much turning traffic expected at each intersection)
– Expected future redevelopment around the site, including the potential for a future four-legged intersection at these access points
– Long-term future volumes expected on the County Roads
– Expected vehicle delays and stacking with and without the turn lanes

Eventually, it was agreed that left and right turn lanes on the mainline would be provided at one intersection without turn lane improvements at the other. This compromise position provided the desired turn lanes at the primary intersection, where 2/3 or more of the development traffic is expected, while recognizing the cost of turn lanes at the other intersection would be more than the potential benefit.

As stated, the decision on when to add turn lanes can be complicated. Be prepared to examine multiple factors and collaborate with other traffic engineers in reaching a final decision.