The accident would have been 100% avoidable.

Ach, geht es auch nur zu 93,5%?

Ich würde einfach sagen

The accident would have been avoidable.

wollte ich auch gerade schreiben^^

Alternative to Helmi's suggestions:

The accident would have been clearly avoidable.

The accident shouldn't have occurred in the first place.

You can slice it, dice it and puree it, but since it's a sentence from an accident report one should stick with the German. :-))

@Helmi (U.S.) -- I'm putting away my slice and dicer, but not my pureer. (sp.?) ;-)
You're right. For an accident report, the 100% should remain.

--> Good call (#1).