Serving Northern St. Louis County, Minnesota

Sherman’s Corner redesigned for safety

But MnDOT’s own study suggests accidents may increase with the change

Marshall Helmberger
Posted 10/9/19

ANGORA — The intersection here, long known as Sherman’s Corner, is getting a $1 million redesign in an effort to improve traffic safety, according to the Minnesota Department of Transportation. …

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Sherman’s Corner redesigned for safety

But MnDOT’s own study suggests accidents may increase with the change

Posted

ANGORA — The intersection here, long known as Sherman’s Corner, is getting a $1 million redesign in an effort to improve traffic safety, according to the Minnesota Department of Transportation.

The new intersection, at the junction of the Hwy. 53 expressway and Hwy. 1/County Rd. 22, is designed to prevent traffic from directly crossing Hwy. 53 in hopes of stemming crashes at the intersection. MnDOT manager on the project, Josie Olson, said the crash rate at the site has increased somewhat since the expansion of the highway to four-lane in 2014.

The new design, known as a “conflict-reduction intersection,” will require drivers seeking to cross or turn left onto Hwy. 53 to turn right onto the expressway, then merge left before making a U-turn a hundred yards or so down the road, then traveling back to the intersection across the highway from their starting point, where they will either continue on their way down the expressway or turn right again to continue on their way.

While traffic records show an increase in crashes at the intersection since the expansion of the highway to four lanes, the increase is not dramatic. In a six-year period, from 2009-2014, the intersection saw 11 crashes, including seven with injuries, one severe. In the four-and-a-half years since completion of the four-lane, the intersection has seen 12 crashes, including six with minor injuries. MnDOT traffic engineer Morrie Luke said the new designs should reduce crashes, particularly dangerous side impacts, leading to fewer injuries and deaths. The innovative design has been used elsewhere in the country, and with increasing frequency here in Minnesota. MnDOT’s Olson said the state now built close to 80 such intersections, mostly where rural highways cross high-speed expressways.

A recent study by MnDOT suggests that reduced-conflict intersections can improve traffic safety at some rural four-lane intersections. Yet that same study found that crash rates actually increased, in some cases, significantly, at half of the eight intersections that MnDOT examined both before and after installation of a conflict-reduction intersection.

The overall statistics cited in the study suggest significant reductions in the severity of injuries as a result of the intersection redesign. The study examined the number and types of crashes at eight intersections where a two-lane road crossed a four-lane expressway, both before and after the installation of a reduced-conflict intersection. The total number of crashes related to the intersection declined from 53 prior to installation of the new intersection, to 45 post-construction. But MnDOT officials say, more importantly, the number of high-speed side impact, or “T-bone,” accidents that can occur at traditional intersections, are substantially reduced with the new design. Those side impacts are the most common type of fatal or severe-injury accident that occurs at such intersections.

The study confirmed that these side impact crashes are significantly reduced, at least in most cases. Overall, the study concluded that right angle crashes are reduced by 77 percent and that fatal accidents are reduced by 100 percent.

But such statistics don’t necessarily predict the outcome at every newly-installed reduced-conflict intersection, and the claimed safety benefits are overwhelmingly due to significant reductions in crashes at just two busy intersections, which had accounted for the bulk of the crashes in the study prior to the redesign of the two intersections. Most of the remaining intersections either saw no statistically-significant benefit, or saw crashes increase as a result of the redesigned intersection.

The redesign of the intersection of Hwy. 53 and County Rd. 52 in Cotton is an example. In a three-year period prior to the installation of a reduced-conflict intersection there, drivers had experienced three crashes at the site, two of which had involved personal injury and one of which had involved only property damage. In the three years after installation of the new intersection, drivers at the site experienced seven crashes. As before, two had involved personal injury, with the remaining involving damage to vehicles.

The experience Cotton was not unusual. Indeed, four of the eight intersections studied saw increases in crashes, and three of those actually saw slight increases in right angle or T-bone crashes and personal injuries following the redesign of the intersection. A fifth intersection saw no change in either the number of crashes or injuries. That means that of the eight intersections studied, just over 60 percent saw an increase in crashes or saw no improvement.

So how did the MnDOT study’s overall statistics show such dramatic safety improvements?

Two intersections accounted for the vast majority of the documented gains in crash reduction and the rate of injuries, none more so than one located near Willmar. In a five-year period prior to the study, the intersection had seen 17 crashes, including ten with personal injuries, including one fatality. After redesigning the intersection, drivers experienced just two crashes in the five years following reconstruction, with none involving personal injuries.

The results clearly indicated that the changes made at the site improved traffic safety, and they undoubtedly affected the results of the study. Simply removing this single outlier from the results of the study, and the installation of reduced conflict intersections would have shown an increase in crashes from 38 prior to their installation to 45 after installation.

While the number of crashes involving personal injury would have still shown improvement, the numbers would have been far less dramatic without the Willmar intersection. A second redesigned intersection, near Cologne in Carver County, accounted for virtually all of the remaining improvement in personal injuries demonstrated in the study. In a three-year period prior to installation of the new intersection, the crossing had seen 15 crashes, including three fatalities and six others with personal injuries. Following redesign, the site experienced no fatalities and only two accidents with minor injuries. Overall crashes dropped only slightly, from 15 prior to redesign, to 12 post-reconstruction, yet all but two resulted only in property damage.

The improved traffic safety at those two locations overwhelmed the combined results from other intersections in the study, even though most sites saw more crashes or no change as a result of the intersection redesign.

Low traffic count

Compared to most other reduced conflict intersections studied by MnDOT, the traffic counts at Sherman’s Corner are extremely low, barely one-fifth the level of traffic at the Cotton intersection studied by MnDOT. The installation of the four-lane has had little impact on traffic counts along Hwy. 53, with about 3,800 cars per day traveling on that section of highway. About 550 cars per day enter the intersection from Hwy. 1/County Rd. 22, according to MnDOT’s Luke.

While that might raise questions about the cost-benefit of the improvements, Luke said the low traffic count at the crossing makes it potentially more dangerous. “People tend to take more risks when there’s a lower count,” he said. At the same time, he said, local drivers can become complacent because they are used to seeing little traffic and may not look as carefully as they should.

The intersection was built under contract with KGM. Work wrapped up on the project last week.

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