Speed limits: If it acts like a truck, it’s a truck

Q: When we drive our 45-foot motorhome on the freeway, are we supposed to be going the car speed limit or the truck speed limit?

A: The truck speed limit sign is a little misleading; or if not misleading, at least incomplete. Of course, it’s impossible to put the full state law on a street sign so I suppose it’s the best option we’ve got. When you see a speed limit sign that says, “Speed Limit 70, Trucks 60,” the word “trucks” is a stand-in for more than just trucks.

Before we get to the specific law about speed limits for trucks, let’s take a look at speed limits in general. In the section titled, “Basic rule and maximum limits” state law sets maximum speed limits for basic categories of roads:

• City and town streets: 25 mph

• County roads: 50 mph

• State highways: 60 mph

Most certainly, you’ve driven on roads that don’t match those speed limits. The law has provisions for altering speeds, up or down, as appropriate. We have city streets with 20 mph school zones and 30 mph arterials, counties with default 35 mph limits, and 70 mph highways. You can probably think of many other examples in your community. On some routes that I regularly drive there are more roads with adjustments to the maximum limits than there are roads that conform.

It’s not a free-for-all though. Speed limit changes, faster or slower, are determined “upon the basis of an engineering and traffic investigation” making sure that the speed limit is safe and appropriate for the roadway. Traffic engineers adjust limits to balance safety and efficiency.

The law authorizing increases in speed limits has a few limitations. One is a maximum limit of 60 mph for “trucks.” The word “trucks” on the speed limit sign means any vehicle over 10,000 pounds gross vehicle weight and all “vehicles in combination.”

You might think that 10,000 pounds is a lot, and just by the sheer weight self-selects for commercial vehicles. That may have been the case back in the 1960s when we started building interstates with speed limits over 60 mph, but like the average American, vehicles have gained a lot of weight in the past few decades.

For example, a 1965 Ford F350 weighed around 4,000 pounds. A 2022 Ford F350 can weigh nearly double that, and it has a gross vehicle weight rating (the combined weight of the vehicle and the load it’s capable of carrying) of over 12,000 pounds. You could easily have a pickup that, when loaded, would be required to be driven at the posted speed limit for trucks.

As you might have guessed, lots of motorhomes well exceed the 10,000-pound mark. The big class A RVs can hit 50,000 pounds. Even some van conversions are over 10,000 pounds. If your RV is a trailer, the law also applies to you, regardless of weight. Earlier I referenced “vehicles in combination.” That’s the term the law uses to refer to all vehicles towing a trailer.

Why do we limit the speed of heavy vehicles? Physics. Braking power required to stop a vehicle depends on the vehicle weight and the square of its speed. Even with bigger brakes on bigger vehicles, there comes a point where heavy vehicles just can’t stop as fast as lighter ones. Commercial drivers learn about weight and stopping distance; for the rest of folks driving RVs or other heavy vehicles, it’s good to keep in mind that your stopping distance is different than when you’re driving your car. Choose your speed and following distance accordingly.

Doug Dahl writes a weekly column for this newspaper. He is the Target Zero manager for communications with the state Traffic Safety Commission.