I would argue that it is. Alright, it does have action elements to in the form of the shooting and driving, but it isn't an action/adventure: That makes it sound like Uncharted. I think L.A. Noire is an adventure game because the driving force behind the whole game is the storyline, and the majority of gameplay is investigating crime scenes and interrogating suspects.
What are your thoughts? Personally L.A. Noire is one of my favourite games from the past few years: Quite an amazing experience with incredible attention to detail.