Why Semi-Trucks Can Now Drive 80 mph on Idaho’s Interstates

Drivers sharing Interstate 84 with big rigs west of Caldwell and east of Boise are now sharing the road with trucks moving as fast as they are. Idaho eliminated its long-standing split speed limit on July 1, letting heavy trucks over 26,000 pounds travel up to 80 mph on qualifying stretches of interstate, the same limit passenger vehicles already use.

The change came from House Bill 644, signed by Governor Brad Little in March, and it makes Idaho one of a shrinking number of states to fully close the speed gap between cars and commercial trucks. The Idaho Trucking Association says its members have “grave” safety concerns, even as the bill’s sponsor argues the old system was the real danger.

What Changed And Where

For more than a decade, Idaho ran a split speed limit system: passenger cars could travel 80 mph on rural interstate stretches, while trucks at 26,000 pounds or more topped out at 70 mph. That 10 mph gap dates back to 1998, when the state first separated truck and car speeds on its highways.

HB 644 erases that gap. Starting July 1, the Idaho Transportation Department began removing truck speed limit signs on the affected corridors, a process expected to take about a day. In the Treasure Valley, the new uniform limit applies to Interstate 84 west of Caldwell and east of Boise. Farther east, it also covers sections of Interstate 86 and Interstate 15. Speed limits inside urban areas like Boise remain unchanged at 65 mph for all vehicles, truck and car alike.

Idaho Transportation Department’s board authorized the new uniform limit in a tight 3-2 vote, a split that reflected real disagreement even among the officials tasked with implementing the law. Board chair Bill Moad said plainly he disagreed with the change: “I understand we’re not having a chance to appeal this, but 80 miles an hour will not solve the differential between cars and trucks.” Board member Paul Franz went further, warning that faster trucks “makes trucks tip over, and when they do, they wipe out, usually taking other traffic with them.”

The Case For Ending Split Speeds

State Rep. Doug Pickett, R-Oakley, sponsored HB 644 after describing what he called a fatal problem on Idaho’s highways: differential speeds, not raw speed itself. Pickett cited a 2005 University of Arkansas study finding that vehicles traveling slower or faster than the surrounding traffic flow are far more likely to interact dangerously with other vehicles. One finding from that research: a vehicle traveling 10 mph below the posted limit sees a 227 percent higher rate of these interactions compared with a vehicle keeping pace with traffic.

“Speed doesn’t kill,” Pickett told colleagues during a House Transportation Committee hearing in February. “Differential speed kills.”

Pickett also pointed to a common highway frustration as part of his reasoning: two trucks traveling side by side at a slower speed, blocking both lanes and forcing a long line of cars to wait for an opening to pass. He described that scenario as a safety hazard in its own right, not just an inconvenience, and argued a uniform speed limit would reduce how often it happens. Pickett placed Idaho’s old system within what he called a national trend away from split speeds, estimating that Idaho was one of 10 or fewer states still running them before HB 644 passed.

Why Truckers Are Worried

Allen Hodges, president of the Idaho Trucking Association, points to different data than Pickett does. He cites Idaho Transportation Department figures suggesting that crashes between commercial trucks and passenger cars on higher-speed highways can produce more fatalities and injuries, not fewer. His core physical argument is about stopping distance: heavier trucks need more room to stop, and that room grows fast as speed increases.

“Idaho is only one of two states that the federal government allows 129,000-pound loads daily on the freeway system,” Hodges said. For the heaviest trucks permitted on Idaho roads, he estimates the new 80 mph limit adds nearly 200 feet of additional braking distance compared with 70 mph.

There is also a tire and equipment issue few drivers think about: most commercial truck tires carry a manufacturer speed rating capped around 75 mph. Hodges says more than 70 percent of trucking association members surveyed say they have no plans to actually increase their driving speed under the new law, regardless of what the posted limit now allows. Many fleets also run speed governors that cap a truck’s top speed well below any posted limit, for both fuel economy and safety reasons that predate HB 644 entirely.

Hodges said the association offered lawmakers an alternative: restrict large trucks from traveling in the left lane on rural interstates, addressing the side-by-side blocking problem Pickett described without raising the speed ceiling. “It fell on deaf ears,” Hodges said.

What Drivers Should Know Sharing The Road

Idaho State Police spokesperson Aaron Snell has a direct message for everyday drivers adjusting to the change: don’t assume trucks are moving at any particular speed. “Drivers may see commercial vehicles traveling at higher speeds than they are used to, particularly on interstate and rural highways,” Snell said, while cautioning that the law changing posted limits does not change the physical realities of operating a large commercial vehicle.

“Trucks require greater stopping distance, have significant blind spots, and may travel below the posted limit due to equipment, loads, or road terrain,” Snell said. “Drivers should not assume traffic will move uniformly and must continue to drive attentively and responsibly.”

That guidance cuts both ways for passenger vehicle drivers on the affected corridors. A truck now legally permitted to travel 80 mph might still be moving well under that speed, held back by its load, its tires, or a governor limiting its engine, so drivers should not expect uniform truck speeds even where the new limit applies. At the same time, a truck that is running at 80 mph needs meaningfully more distance to stop or slow down than it did under the old 70 mph cap, a fact worth remembering before cutting closely in front of one to exit or change lanes.

How Idaho Compares Nationally

Idaho joins a small group of states without any speed differential between passenger vehicles and heavy trucks on their fastest highways. Texas, for example, allows both cars and trucks to travel 75 mph or higher on many rural interstates without a split system. Most states still separate the two, often by 5 to 10 mph, reflecting the same stopping-distance concerns Hodges raised about Idaho’s new law.

The debate playing out in Idaho mirrors a broader national argument in highway safety circles about whether speed uniformity or lower truck speeds does more to prevent crashes, and the state’s real-world crash data over the coming year will likely become a reference point either side cites in future legislative fights elsewhere.

Looking ahead, several open questions remain. The Idaho Transportation Department has not announced a formal review period or specific crash-tracking benchmark tied to HB 644, unlike some states that build in automatic sunset reviews when changing commercial speed rules. That leaves Idaho’s actual outcome, whether it reduces the tailgating and pass-and-block scenarios Pickett described, or produces the higher-speed crash severity Hodges and ITD’s own board members warned about, as an open question that will play out in real crash reports over the coming months.

For now, drivers on I-84 west of Caldwell, east of Boise, and on the affected sections of I-86 and I-15 should treat the removal of truck speed limit signs as confirmation that a nearby semi is no longer bound by the old 70 mph ceiling, not as a guarantee that every truck they pass will actually be moving that fast. Idaho State Police’s core advice, driving attentively and leaving extra room around large commercial vehicles, applies whether a truck is running at 65 mph or 80 mph under the new law.

Sources:

• https://www.spokesman.com/stories/2026/jun/21/heavy-duty-trucks-can-soon-go-80-on-some-idaho-fre/
• https://www.kmvt.com/2026/06/25/idaho-allows-heavy-trucks-drive-80-mph-starting-july-1/