In some parts of the US, the word "highway" is used to refer to roads (sometimes quite wide) that have stop lights and with speed limits around 40 MPH. I grew up in northern California, and we call roads without stop lights "freeways" and roads with stop lights "highways". There are various other terms, of course (e.g. "expressways"), but these are the most common generic descriptors.

I wouldn't assume that the driver going slowly was much under the speed limit, if at all.

Which is understandable when you consider there were "highwaymen" in the 1600s when the highways had nothing faster than a horse.

I should have clarified. 4-lane expressway, signals were about 0.5-1.0 mile apart. All lanes clear. Mid night.

It was NOT a freeway.

Makes sense, the usage is colloquially different in the Midwest.

Maybe going up a hill? They did mention they were driving a truck, it’s easy to have vastly different climbing speeds in that case.

Also, I would often go less than 20 down a pass during the winter because I didn’t have snow tires or chains, and was passed by plenty of locals doing 60+ because they were equipped for it.

Yeah, that seems dangerous. There is usually a hard 45mph minimum on the highways near me.

I'm never going to to discourage people from driving slow, because our monkey brains have shit reaction times and perception, and KE increases with the square of velocity, but if you're traveling well below the normal speed of travel, put your flashers on. In many places this is mandated for a certain amount under the limit.

In my view the wreck isn’t made worse so much by absolute magnitude of one momentum vector as by the magnitude of the difference in momentum vectors.

You have to consider that after a wreck, both cars have to come to a stop one way or the other.

Imagine being rear ended while you are stopped at a stop sign. The difference in momentum is small and there is no significant damage.

Now imagine going 80mph down the highway and someone going 81mph hits the back of your car. Still a very small difference in momentum, but your car is very likely to lose control and roll or hit other cars. It's not always the initial impact that does all the damage.

To put it another way, every collision you are in is also a collision with the Earth, which masses a lot more than you and wins every exchange of momentum.

Conveniently, your delta-v with this massive body you don't want to run into is also the one conventionally reported for your speed.