MI HB4012

Establishes that speed limits must be determined using the 85th percentile speed of free-flowing traffic under ideal conditions, set in 5 mph increments within 5 mph of that percentile speed.

Allows speed limits to be set below the 85th percentile speed if an engineering and safety study demonstrates hazards to public safety, but prohibits setting limits below the 50th percentile speed.

Requires traffic control orders establishing modified speed limits to be filed as public records with the county clerk (for freeways/trunk lines) or city/village clerk (for local roads) and makes certified copies admissible as evidence in court.

Permits local road authorities to determine modified speed limits on local highways in accordance with traffic engineering practices, and requires signs to conform to the Michigan manual on uniform traffic control devices.

Defines key terms including "design speed" (per American Association of State Highway and Transportation Officials standards), "local road authority," and "traffic control order."