It takes two weeks or so to learn your driving patterns (after installing iOS 7), and should start showing traffic information during the 3rd full week.
Do you have a regular commute each weekday?
Also, if you reset the settings on your phone, or have to restore it, you will lose the built-up driving patterns that the phone has collected as this information is not stored in iCloud or backed up to iTunes. So if you've done either of those, it starts the phone's learning all over again.