1. He was driving at 70 km an hour.
2. He was driving at 50 miles an hour.
3. He was driving at low speed.
4. He was driving at full speed.
(Are they all grammatical? Do you have some other expressions about "at _____"?
All are correct, yes.
You can also use "at" when referring to place and time.
She will arrive at my house around noon.
She will be here at noon.
He will get to his office at 8:00 a.m.
Yes, all four examples are grammatical. However, the third and fourth examples, "He was driving at low speed" and "He was driving at full speed," could be improved by using the adverb "a" before "low" and "full," respectively. This creates a clearer and more idiomatic sentence structure.
Regarding other expressions with "at _____," here are a few examples:
1. He was driving at a moderate pace.
2. She was typing at lightning speed.
3. They were playing chess at a professional level.
4. The plane was flying at a high altitude.
5. The dog was barking at an approaching stranger.
In these examples, the phrase "at _____" is used to describe various aspects such as speed, pace, level, altitude, or the target of an action.