Development Type: Redevelopment Retail Site
Services Provided: Traffic Impact Study
Client: Tri-Land Properties, Inc.
One of the toughest traffic engineering decisions is when it is appropriate to provide mitigation on a two-way, or side-street, stop-controlled intersection. For other types of intersection control, the level of service (LOS) result provides a good indication of operations and clues about what to do next, if anything. For two-way stop-control, LOS is often misleading and can result in recommended expensive changes that may not necessarily be needed.
In the City of Burnsville, redevelopment of the Burnsville Marketplace created such a situation. Currently approved and underway, the site redevelopment would update a grocery store and add up to about 90,000 square feet of additional outlot retail development. Working for Tri-Land Properties, Inc., our task was to examine the surrounding access and roadways in a standard Traffic Impact Study. With the expansion of retail uses on the site, any requirements for access were a primary concern for both the developer and the City.
Per our review of the site, one intersection become a focal point. This existing full access intersection to a County road provided acceptable operations under today’s traffic. In the future, the LOS for the access movements, particularly the left turn, onto the County road dropped from acceptable to unacceptable. If that was our only metric, we would have recommended changes like potentially limiting the access to ¾-access or right-in/right-out only.
As many intersections have poor LOS during the peak periods, the vehicle stacking has become our ‘go-to’ metric for side-street stop control. We examined the 95th percentile vehicle queues to determine how extensive the poor LOS really is. Based on our prior experience, less than five vehicles queued suggests the intersection does not have sufficient traffic to justify the cost for mitigation. Above ten vehicles stacked suggests mitigation of some form is justifiable. Between five and ten vehicles in the 95th percentile queue is the gray area that should also rely on only factors.
For this project, the access intersection of concern had a 95th percentile queue of only three or four vehicles during the peak hours. Although this result is clearly below our threshold, we further examined other factors that could influence the decision, including:
+ ‘T’-intersection providing access to a private development
+ Very close to an existing signal
+ Other access intersections available to reach the same location, although longer in distance
+ Vehicle queueing occurs within the development, not on the public road, and would not interfere with normal operations
+ The peak hour vehicle queues occur during one hour of the day with the other 23 hours having less stacking
In general, traffic engineers should look beyond the basic LOS results, especially when the results are close to the acceptable/unacceptable level. The LOS of side-street stop control intersections has even less meaning in our opinion and consideration of other factors is critical to determining the most appropriate course of action. For this project, that meant we recommended against mitigation and ensured existing cost and future maintenance were not expended unnecessarily.