Idaho Modernizes Roadways: Eliminates Truck Speed Differential for Enhanced Safety and Efficiency

Starting July 1, 2026, Idaho will let big trucks drive at the same speed as regular cars on all highways. Before, trucks had to go 10 miles per hour slower than cars, but a new law will change that. The goal is to make roads safer and traffic smoother, so there are fewer problems with cars passing slow trucks. Many other states have already made this change because studies show it helps prevent accidents and makes driving less stressful for everyone.

Idaho will let big trucks drive at the same speed as regular cars on all highways.

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What is changing with Idaho's truck speed limits in 2026?

Starting July 1, 2026, Idaho will eliminate the 10 mph speed limit differential between heavy commercial trucks and passenger vehicles. Trucks will be allowed to travel at the same speeds as cars on all Idaho highways, improving safety, reducing congestion, and streamlining traffic flow statewide.

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Idaho's roadways are set for a significant change as House Bill 664 has been signed into law, eliminating the long-standing speed limit differential between passenger vehicles and heavy commercial trucks. The legislation, which Governor signed on March 24, 2026, will take effect on July 1, 2026, fundamentally altering how large trucks operate on the state's highways.

Understanding the Speed Limit Change

For years, Idaho law required vehicles with five or more axles operating at a gross weight exceeding 26,000 pounds to travel 10 mph slower than passenger vehicles on certain roadways. This differential applied primarily to interstate highways in nonurban areas, where trucks were capped at lower speeds while cars could travel faster. House Bill 664 removes this requirement, allowing heavy trucks to match the speed limits of lighter vehicles across all road types - interstates, state highways, and urban areas alike.

The bill amends Section 49-654 of the Idaho Code, standardizing maximum lawful speeds regardless of vehicle weight or axle configuration. This means that where passenger vehicles can travel 75 mph on rural interstates, heavy trucks will now be permitted the same speed, rather than being restricted to 65 mph as before.

The Legislative Journey

Legislative Milestone	Date
Committee Referral	February 16, 2026
Signed into Law	March 24, 2026
Effective Date	July 1, 2026

Industry representatives indicated that state law would likely require the Idaho Department of Transportation to conduct a safety review before the speed limit changes take effect, though specific details of such a review have not been publicly disclosed.

Safety Implications and National Context

The debate over speed differentials between trucks and passenger vehicles has intensified nationwide in recent years. Research from multiple states suggests that eliminating these differentials can improve highway safety by reducing the frequency of passing maneuvers and vehicle interactions.

A 2019 joint study by the Indiana Department of Transportation and Purdue University found that raising truck speed limits to match passenger vehicles on interstates would reduce crash frequency by approximately 20%, primarily because fewer cars would need to overtake slower-moving trucks.

The logic behind uniform speed limits centers on traffic flow dynamics. When all vehicles travel at similar speeds, there are fewer opportunities for dangerous passing situations, lane changes, and the accordion effect that creates congestion. A 2005 University of Arkansas study demonstrated that speed differentials increase vehicle interactions and accident likelihood on rural interstates, as cars pass slower trucks more frequently and at higher relative speeds.

The Nationwide Trend

Idaho joins a growing number of states that have eliminated or reduced speed differentials for commercial trucks. Over the past two decades, most states have moved away from differential speed limits based on mounting evidence that uniform speeds improve safety outcomes. Currently, only seven to nine states maintain such differentials, down from widespread use in the 1980s.

Uniform speed limits allow traffic to flow more freely, preventing bottlenecks created when slower-moving trucks impede faster passenger vehicle traffic, reducing overall highway congestion and improving travel times for all motorists.

Michigan's Department of Transportation, for instance, opposed federal speed limiter mandates in 2024, warning that forcing slower truck speeds would increase speed differentials and crashes between heavy and light vehicles. This position reflects the broader shift in transportation policy toward recognizing the safety benefits of uniform speeds.

Traffic Flow and Economic Considerations

Beyond safety, the elimination of speed differentials addresses practical concerns about traffic congestion and commercial efficiency. When trucks travel significantly slower than surrounding traffic, they can create rolling roadblocks, particularly on two-lane highways where passing opportunities are limited. This phenomenon frustrates motorists and can lead to aggressive driving behaviors as impatient drivers attempt risky passing maneuvers.

For the trucking industry, uniform speed limits mean more predictable travel times and reduced citations for drivers who previously faced the difficult choice of either impeding traffic flow or exceeding their designated speed limit to keep pace with surrounding vehicles. The change also simplifies compliance for interstate carriers operating across multiple states with varying regulations.

Implementation Timeline

With the July 1, 2026 effective date approaching, Idaho transportation officials and law enforcement agencies have several months to prepare for the transition. This implementation period allows time for public education campaigns, signage updates, and coordination with trucking companies operating in the state.

The phased approach provides an opportunity to monitor early results and address any unforeseen challenges that emerge as drivers adjust to the new speed regime. Transportation researchers and safety advocates will likely watch Idaho's experience closely, as it adds to the growing body of evidence about the real-world impacts of eliminating truck speed differentials.

Looking Forward

Idaho's decision to eliminate the 10 mph speed differential represents a significant policy shift grounded in evolving research about highway safety and traffic management. As the July 2026 implementation date approaches, the state will join the majority of jurisdictions that have concluded uniform speed limits serve the interests of safety, efficiency, and practical highway operations better than maintaining separate speed regimes for different vehicle classes.