Probably both are possible.

The sentences are indirect quotations, which are easiest to understand if you relate them to what was originally spoken. If what he said was "I see the accident" (he sees it as it is happening), the indirect quote corresponding is "He insisted he saw the accident", where the change from "see" to "saw", called time-shifting, is to make the verb agree in tense with the tense of "insisted".

If what he said was "I saw the accident", then there is some choice about the report of this in indirect speech. In a relatively conservative dialect like mine, the report would be "He insisted he had seen the accident", where the perfect "have" of "had seen" corresponds to the past tense of "saw" in his earlier speech, and the past tense of the "had" corresponds to the past tense of "insisted". This is sometimes called a past within a past, since the past time at which he did the seeing was previous to the past time at which the insisting took place.

In a less conservative dialect, or less formally, you get a simple past tense in the indirect quotation.

There are other cases where a logical past tense is realized as perfect "have" when the past is in a grammatical position that does not permit an ordinary morphological past tense. For instance, "He may have left" means that it is possible he left. But you can't say "He may left" with a simple past tense, because that is not allowed after a modal auxiliary, like "may".

Probably both are possible.

The sentences are indirect quotations, which are easiest to understand if you relate them to what was originally spoken. If what he said was "I see the accident" (he sees it as it is happening), the indirect quote corresponding is "He insisted he saw the accident", where the change from "see" to "saw", called time-shifting, is to make the verb agree in tense with the tense of "insisted".

If what he said was "I saw the accident", then there is some choice about the report of this in indirect speech. In a relatively conservative dialect like mine, the report would be "He insisted he had seen the accident", where the perfect "have" of "had seen" corresponds to the past tense of "saw" in his earlier speech, and the past tense of the "had" corresponds to the past tense of "insisted". This is sometimes called a past within a past, since the past time at which he did the seeing was previous to the past time at which the insisting took place.

In a less conservative dialect, or less formally, you get a simple past tense in the indirect quotation.

There are other cases where a logical past tense is realized as perfect "have" when the past is in a grammatical position that does not permit an ordinary morphological past tense. For instance, "He may have left" means that it is possible he left. But you can't say *"He may left" with a simple past tense, because that is not allowed after a modal auxiliary, like "may".