Charles Ellicott Commentary Exodus 20:18

"And all the people perceived the thunderings, and the lightnings, and the voice of the trumpet, and the mountain smoking: and when the people saw it, they trembled, and stood afar off." — Exodus 20:18 (ASV)

And all the people saw the thunderings

— That is, they perceived them. On the true character of the Sinaitic manifestation, see Note on Exodus 19:16-20.

They removed. — Moses had brought the representatives of the people as near to Sinai as possible—close to the foot of the great precipice of Ras Sufsâfeh (Exodus 19:17). The wide plain of Er-Rahah allowed them to move to a considerable distance.