Bill
HB 4137
Allows local authorities to set posted speed limits up to 5 mph below the 85th percentile without a full engineering and safety study.
Status & origin
- Introduced March 10, 2025 (Rep. Bradley Slagh). Read first time March 27, 2025; referred to Committee on Transportation and Infrastructure. House Fiscal Agency analysis completed to 4-14-25. Bill would amend section 628 of the Michigan Vehicle Code (MCL 257.628).
- Note: the packet provided also includes an unrelated Illinois bill using the same bill number; this summary focuses on the Michigan Vehicle Code amendment text and the House Fiscal Agency analysis.
Purpose / intent
- To modify the procedures and standards for establishing posted speed limits on Michigan highways so local authorities may set lower posted limits up to a modest margin below the 85th‑percentile speed without conducting a formal engineering and safety study.
Background
- Current law uses traffic‑engineering practices and the 85th‑percentile speed of free‑flowing traffic under ideal conditions as the basis for most modified posted speed limits. Modified limits must be in 5‑mph increments within ±5 mph of the 85th‑percentile speed, and cannot be set below the 50th‑percentile speed. Lower limits may be set below the 85th percentile only if an engineering and safety study shows hazards not reflected by the 85th‑percentile speed.
Key provisions (what the bill changes)
- Allows a modified speed limit to be set up to 5 mph below the 85th‑percentile speed without requiring an engineering and safety study.
- Maintains that any reduction greater than 5 mph below the 85th‑percentile speed requires an engineering and safety study demonstrating hazards to public safety that are not reflected by the 85th‑percentile speed.
- Preserves the prohibition on setting posted limits below the 50th‑percentile speed.
- Keeps other existing procedural elements in section 628: authorities empowered to set limits (county road commissions, township boards, MDOT and MSP for trunk lines/limited access freeways, local road authorities for local highways), requirement to file traffic control orders as public records, requirement that posted signs conform to the Michigan Manual on Uniform Traffic Control Devices, and that violations are civil infractions.
- Clarifies advisory signage and treatment when a segment contains features with lower design speeds.
Who would be affected
- Local road authorities (county road commissions, township/city/village boards, MDOT, MSP): gain limited additional discretion to lower posted speeds by up to 5 mph below the 85th percentile without performing full engineering studies.
- Motorists and enforcement: potential for more posted limits slightly below prevailing free‑flow speeds; enforcement remains civil infraction-based.
- Local governments: expected minimal or no direct fiscal impact (no new procedural costs anticipated per House Fiscal Agency).
Procedural / fiscal notes
- House Fiscal Agency: no anticipated direct fiscal impact on state or local units because the bill does not create materially new study requirements.
- The change alters the threshold at which a formal engineering and safety study is required (only needed when proposed limits exceed 5 mph below the 85th percentile).
References
- Amends MCL 257.628; relates to MCL 257.627 (Basic Speed Law). House Fiscal Agency analysis (complete to 4-14-25). Michigan State Police and MDOT guidance on establishing speed limits referenced in the analysis.
Compiled from official sources — confirm details with the bill’s official record.
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