In Idaho, setting or altering a speed limit is a highly regulated, data-driven process. It isn’t based on guesswork or a sudden influx of resident complaints; rather, it follows a strict protocol dictated by Idaho Code and federal guidelines.
The authority is split between the Idaho Transportation Department (ITD) (for interstates and state highways) and local authorities like city councils or highway districts (for local roads).
The specific sequence of steps required to legally establish or change a speed limit in Idaho unfolds as follows:
1. Initiation & Jurisdiction Identification
A review is triggered either by a local city’s request, ITD’s routine corridor monitoring, or a citizen petition. The first task is confirming who owns the road. By Idaho law, ITD sets limits on state highways and interstates (even where they cut through city limits), while local councils or county highway districts manage local residential and business streets.
2. The Engineering & Traffic Investigation
Idaho Code (§ 49-201 and § 49-207) strictly forbids changing a statutory speed limit without a formal traffic engineering study. ITD or local engineers go to the field to assess the road’s physical and environmental design, documenting:
- Lane widths, pavement type, and road conditions.
- Terrain (hills, sharp curves, blind spots).
- “Conflict points” like the density of commercial driveways, intersections, and street-parking setups.
- Historic crash data and frequency.
3. Calculating the 85th Percentile Speed
Engineers conduct a radar or sensor study to measure the speeds of free-flowing traffic under ideal weather conditions. In accordance with federal safety standards, Idaho uses the 85th percentile speed as its baseline. This is the speed at or below which 85% of drivers naturally travel. Traffic engineering philosophy holds that the majority of drivers are inherently attentive and self-regulate to a safe speed based on the road environment.
4. Reviewing Statutory & Uniformity Constraints
The proposed limit is cross-referenced with maximum caps set by Idaho law (e.g., maximums of 80 mph on rural interstates, 65/70 mph on state highways, and 35 mph in urban districts). Engineers also apply uniformity rules. For example, Idaho legally eliminated “split speed limits,” meaning commercial trucks are required to match the speed limit of passenger vehicles on major routes to minimize dangerous lane-weaving and speed variances.
5. Official Approval & Ordinance Passage
If the data warrants a change, it must be legally codified. For local roads, the city council or county commissioners must vote on and pass an official ordinance during a public meeting. For state roads, the regional ITD engineers present the data to the Idaho Transportation Board for formal authorization.
5. Sign Installation & Law Enforcement Notification
A speed limit is legally unenforceable until proper signage is visible to drivers. Maintenance crews are dispatched to install new retroreflective signs adhering to the Manual on Uniform Traffic Control Devices (MUTCD). Simultaneously, local police or the Idaho State Police are formally notified so they can adjust their enforcement parameters.
******* The “Basic Rule” Overrides Everything: Even after a speed limit is legally posted through this process, Idaho Code § 49-654 mandates that no one may drive faster than is “reasonable and prudent” for current conditions. If a road is covered in snow or sheeted in ice, a driver can still receive a citation for driving the posted 65 mph limit.