Why Are Semi-Trucks So Slow On The Highway? Speed Limitations, Explained

While driving along the interstate, it's common to see the bulk of the semi-trucks hauling different types of semi-trailers in one of the slow lanes since they drive considerably slower than the smaller sedans, SUVs, and trucks they share the road with. The posted speed limit throughout the United States can vary with a range of anywhere from 55 miles per hour to 80 mph, and big rigs are typically seen driving near the lower range no matter what. Semi-trucks might drive slower, but they are capable of driving around the same speed as many of the other vehicles on the road with them. However, driving slower is necessary for safety reasons.

The highway can be an unpredictable landscape and the larger the vehicle is, the more dangerous it can be. Semis are always hauling something. There could be a trailer full of logs or consumer products. Either way, it adds a significant amount of weight that can get up to around 80,000 pounds.

A fully-loaded semi driving 65 mph needs 525 feet to come to a safe stop under the best of conditions. A typical sedan or SUV at the same speed needs about 316 feet. If that same semi were to drive faster, it would need more space to come to a complete stop. Additionally, consider that road and weather conditions are rarely ideal, so it's better to drive the trucks slower than the posted speed limit. It's all a matter of physics and one reason semi-trucks have so many gears.

Truckers across the United States have been waiting for the Federal Motor Carrier Safety Administration to make a decision on its speed limiter mandate, which would require the largest semi trucks and all vehicles that exceed 26,001 pounds gross weight to install an electronic control unit (ECU). The ECU would be programmed with a specific speed limit, preventing the vehicle from exceeding it. There's no mention of what the speed limit would be yet. A previous proposal stated a top speed of 68 mph, but the document has since been altered and left without any such limit. This has been an ongoing debate since 1991 and even when the safety administration announced its proposal, it has undergone multiple delays. 

The most recent delay has pushed a final decision to May 2025. It's a controversial idea, to be certain. Opponents of the mandate believe it stifles flexibility drivers can currently enjoy, will cause more traffic, and make it more dangerous rather than safer. Meanwhile, supporters of the mandate, such as executives from J.B. Hunt and Schneider, have argued that it will cut down on the number of deadly vehicle accidents each year, according to TruckStop.

Why has the Federal Motor Carrier Safety Administration taken so long to implement its mandate? Those in charge haven't remained in their leadership role for long. Robin Hutcheson resigned from her position as the head of the safety administration in 2024 after being appointed in 2022. There wasn't a full-time administrator for the three years before her.