Have you ever been driving down the road, traveling the speed limit along with everyone else, and then suddenly the speed limit drops by 20 miles per hour? Do you wonder, who in the world decided to reduce the speed limit and why? How are speed limits in Idaho set anyway?

Establishing Speed Limits

Speed limits are set by Idaho law, specifically Idaho Code 49-654. In the table below you will see the maximum speed limit for various types of roadways. These limits may be reached where no special hazards or conditions exist that requires a driver to lower their speed to fit conditions, such as weather or condition of the roadway.

However, Idaho’s roadways can be unique from one another. Therefore, an opportunity exists for state and local highway authorities to change the maximum speed limit for roads within their jurisdiction. According to I.C. 49-207, changes to the set maximum speed limit cannot be done arbitrarily, instead they must be accomplished through a traffic engineering study that is managed by a professional engineer licensed in the State of Idaho. If the engineering study finds that adjusting the maximum speed limit is safe and practicable, then the speed limit may be adjusted within the scope of the law.

Here are the maximum speed limits for roads in the Gem State:

Type of Roadway
Maximum Speed per I.C. 49-6542 Maximum Speed with Engineering Study (I.C. 49-207)
Residential – Urban – Business Districts 35 mph 35 mph or lower
Local Rural Roadways Outside of Residential – Urban – Business Districts 55 mph 70 mph
State Highways 65 mph 70 mph
Interstate Highways (Non-Urban) 75 mph (65 mph for trucks) 80 mph (70 mph for trucks)
Interstate Highways (Urban) 65 mph N/A

It is important to recognize that when a special hazard or condition exists, Idaho Code 49-654 states that vehicle operators shall not “… drive a vehicle at a speed greater than is reasonable and prudent under the conditions and having regard to the actual and potential hazards then existing.” As Brent Jennings, P.E. with Jennings Consulting, LLC explained, this includes approaching and crossing intersections, rail grade crossings, going around curves, approaching hilly terrain, traveling on narrow and winding highways, and when special hazards exist with respect to pedestrians or other types of traffic. This special condition is known as “Basic Rule” and drivers can be cited for violating Basic Rule even though they are driving below the allowed maximum speed limit.

 Did you know?

In Idaho, the speed limit does not have to be posted. However, speed limits are posted to remind motorists of the maximum speed limits.