When Forza Motorsport 5 was first announced at E3, the footage of the game appeared to depict 3D crowds, much as other sports games do.
However, this does not appear to be the case now that Forza Motorsport 5 is out. Instead of being rendered in three dimensions as proper characters, the developers at Turn 10 Studios appear to have gone with stock photographs of people simply milling around like disinterested participants in what ought to be an interesting race.
Here's a comparison image, as taken by a redditor who captured the images and put them side by side:
Click the image for a larger version.