Do you know how much accidents cost? $100M is nothing in comparison over the long term. Plus this was a 10 lane overpass and a unique build during a high inflation period. That is not simple terraforming amigo.

Instead of guessing you could look into the research literature on animal road crossings. Its a surprisingly large area of study because it is hard to get animals to use these constructions. Maybe they did a bad and expensive job, but you have nothing to base that on at all.