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Carver County work session advances 'mumble' strip policy to cut lane‑departure crashes
Summary
County traffic engineers proposed replacing older continuous rumble‑strip rules with newer sinusoidal 'mumble' designs, reducing residential break distances and minimum lengths; staff cited 996 lane‑departure crashes (2016–2025) and a 50–62% crash‑reduction estimate from crash modification factors.
Carver County engineers proposed a significant update to the county's rumble‑strip policy in a work session on April 7, recommending use of newer sinusoidal "mumble" designs, smaller residential break zones and shorter minimum installation lengths to expand where low‑cost safety countermeasures can be used.
"If we can just save one family from not losing a loved one," traffic engineer Whitney Schroeder told the board, "then this update will be successful, and it will be worth it." Schroeder summarized ten years of local crash data (2016–2025) and said there were 996 lane‑departure crashes in that period, 60 of which resulted in fatalities or serious injuries; she cited crash modification studies showing roughly a 50–62% reduction in lane‑departure crashes after installation of appropriate rumble/mumble strips.
Schroeder explained the technical differences between the old rectangular‑grind rumble pattern and a newer sinusoidal "mumble" design that retains vibration inside the vehicle while reducing exterior noise. Field decibel readings presented by staff showed about a 10‑decibel lower exterior reading for the mumble strip pilot on County Road 40 compared with an older rectangular implementation; staff described the mumble pattern as intermittent, with breaks at residences and intersections to mitigate noise. The draft policy would lower the county's current 650‑foot residential break to a proposed 350‑foot radius (based on Local Road Research Board guidance) and reduce the minimum eligible length from 1,350 feet to 150 feet (MnDOT guidance).
Commissioners asked about maintenance, snow‑plow impacts, liability, bicyclist room on shoulders and whether centerline strips should be used; staff said the policy would apply criteria (55 mph or higher, outside city limits, crash history or inclusion in the county road safety plan) and that pilot sites on County Road 40 and County Road 11 have not produced complaints to date. Staff proposed additional outreach to townships and a resident survey on Highway 40, and told commissioners the goal is to bring a final policy for adoption in June (or early August if township outreach delays the schedule).
