The reason why it is にわたる is because it defines what the noun is. 2年にわたる新しい道路の建設工事がやっと終わった This can be separated like this: 建設工事 = Construction work 道路の建設工事 = Construction work of the highway 新しい道路の建設工事 = Construction work of the new highway 2年にわたる新しい道路の建設工事 = Construction work of the new highway that spanned 2 years
So the real noun here is 道路の建設工事, but 「2年にわたる新しい」defines some properties to the noun as well. This grammar is very common when you describe something about a noun. Other examples may be:
「これはネズミを食べる猫です」 This is a cat that eats mice.
You need to have the verb in it's dictionary form to define something about the noun that comes after ^^