Detroit, Michigan — Governor Gretchen Whitmer signed legislation in June 2026 that lets municipalities set speed limits within five miles per hour of the 85th‑percentile speed, rather than being forced to round up to the next five‑mph increment.

The change overturns a decades‑old rule that required a full engineering and safety study before any adjustment and mandated rounding the 85th‑percentile figure up to the nearest multiple of five. For long‑haul drivers and owner‑operators who navigate Michigan’s mix of rural two‑lanes and busy urban arterials, the old system often produced limits that felt out of step with actual traffic flow, adding unnecessary fuel burn and schedule variance.

Rep. Bradley Slagh, the bill’s sponsor, highlighted the new flexibility by citing a road where the measured 85th‑percentile speed was 37.6 mph. Under the old formula the limit would have jumped to 40 mph, but the revised law permits a 35 mph posting, keeping speeds closer to reality. The statute also drops the blanket engineering‑study requirement, allowing adjustments based on standard traffic‑engineering practices. If a study uncovers safety hazards not reflected by the 85th‑percentile, limits may be set below that benchmark, but never lower than the 50th‑percentile speed.

What This Means for Drivers

Truck drivers on Michigan’s interstate system can expect speed limits that better match actual driving conditions, reducing the likelihood of sudden braking or speed‑related citations. CDL‑A drivers hauling OTR loads will see more predictable travel times, which can improve dispatch planning and fuel budgeting. Owner‑operators who rely on tight margins will benefit from limits that avoid unnecessary speed spikes, cutting fuel consumption and wear on tires. The shift also aligns Michigan’s approach with FMCSA regulations that encourage data‑driven safety measures rather than blanket rules.

Industry Reaction

State trucking associations have welcomed the Michigan overhaul, noting that a more nuanced speed‑setting process can ease driver fatigue and lower compliance costs. Some carrier groups argue that the ability to round down will help keep freight moving at steady speeds, especially on routes where traffic density fluctuates seasonally. In New Jersey, the conversation mirrors Michigan’s, with Senator Declan O’Scanlon pushing S1652 to apply the 85th‑percentile formula to the Turnpike and Garden State Parkway. He argues the data‑driven method will produce the “smoothest, safest level of traffic flow” and reduce “arbitrary punishment” for drivers who maintain reasonable speeds. The bill now sits with the Senate Transportation Committee, signaling a possible shift toward evidence‑based limits across the region.

Key Points

• Michigan law now permits speed limits within ±5 mph of the 85th‑percentile figure.
• Local officials can round down, exemplified by a 35 mph limit on a road averaging 37.6 mph.
• Engineering studies are no longer mandatory for every change, though they remain required to set limits below the 85th percentile.
• New Jersey’s S1652 would extend the 85th‑percentile rule to limited‑access highways, pending Senate Transportation Committee approval.

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