Mr. King talked about a new book he has written, named “Mr. Mercedes”. Started as a short story but grew into a 500 page manuscript. Probably not the actual name at publication.
It’s a detective story about a man who has driven into a crowed, but did not get caught. This man contacts a retired detective and starts to challenge him.
Video with explanation at: Stephen King On Twilight, 50 Shades of Grey, Lovecraft & More”