Speed limits are set by the Board of Commissioners based on engineering investigation, and in accordance with Idaho Code, state prescribed methodology and national standards.

Law
Several sections of Idaho Code address speed limits. The main one dealing with setting speed limits is §49-207(3), which states, “Local authorities…shall determine by an engineering or traffic investigation the proper maximum speed…”

Methodology
The methodology for conducting engineering investigations is prescribed in the Manual for Establishing Speed Limits & Speed Zones in Idaho. It includes analysis of the 85th Percentile and Pace Speeds, crash history, and consideration of roadway characteristics, purpose, and usage. For additional information, please see the “Further Reading” articles below. Methods differ, however, for some residential, school, and new construction locations.

Standards
The engineering investigation methodology follows nationally recognized standards, which have been shown by research and experience to result in lowest crash rates and highest safety.
The Manual for Establishing Speed Limits & Speed Zones in Idaho is published by the Local Highway Technical Assistance Council, which is the state agency that provides uniform standards, practices and training on highway and road issues for cities, counties and highway districts in Idaho.

These standards are also used by the Idaho Transportation Department, most other states, and are acknowledged by the following national entities:

• Federal Highway Administration
• American Association of State Highway and Transportation Officials
• American Traffic Safety Services Association
• Institute of Transportation Engineers
• Transportation Research Board

Further Reading. Please see the below articles for more information.

ITD Speed Limits

Michigan Speed Limits

Institute of Transportation Engineers

Washington_State_County_Engineers