Traffic control: speed restrictions; determination in accordance with traffic engineering practices; modify. Amends sec. 628 of 1949 PA 300 (MCL 257.628).

Michigan House Bill 4137 proposes changes to how speed limits are set on county highways. It allows county road commissions, township boards, and the state police to jointly establish speed limits based on engineering studies and traffic conditions. The bill emphasizes using the 85th percentile speed—meaning the speed at or below which 85% of vehicles travel under ideal conditions—to determine appropriate speed limits. These limits must be in multiples of 5 miles per hour and rounded to the nearest 5 mph increment. Additionally, local road authorities are required to document any changes to speed limits and maintain public records of these decisions.

Supporters of HB4137 argue that basing speed limits on the 85th percentile speed aligns with best practices in traffic engineering, potentially leading to safer and more efficient roadways. They believe that this data-driven approach can reduce speed variance among drivers, which is a common cause of accidents. The bill's emphasis on collaboration between local authorities and the state police is seen as a way to ensure that speed limits reflect both local conditions and broader safety standards.

Critics of HB4137 express concerns that relying on the 85th percentile speed could lead to higher speed limits, as drivers often exceed existing limits. They worry that this approach might prioritize driver behavior over community safety, particularly in areas with high pedestrian traffic or near schools. Additionally, some argue that the bill could limit the ability of local communities to set speed limits that reflect their specific safety concerns and preferences.