Having the map is step one. Having cars that actually move is step two. The traffic functionality does not come native to Assetto Corsa. You need a script or a specific mod pack.
It was just a game, but tonight, the drive was real. Assetto Corsa City Map With Traffic Mod
Installing the Assetto Corsa City Map with Traffic Mod is relatively straightforward. Here's a step-by-step guide: Having the map is step one