The central story pushes Driver: San Francisco forward, but there are seemingly unlimited side missions, some of which are story-based, and some are simple "race here", "drive X fast for N seconds", "drift Xm", "Jump X distance in N seconds" and so on.
The side missions are optional, but it is unlikely that you will progress far without completing them. The currency of the game is Will Power - Tanner needs it if he is to wake from his coma - get it. To earn Will Power, you have to perform derring do behind the wheel. Sure you can do this in free-roaming mode. Pretty much everything you do earns WP, drifting, jumping, near misses, even overtaking. But progress this way takes an absolute age. The big points come from completing the side missions.
It's in the acquisition of WP that the game's fine balance becomes apparent. You cannot progress sufficiently quickly through the story without improving certain aspects of your powers. And the storyline missions become locked out unless you complete the city missions. And you simply cannot complete these missions without powering up and having the right vehicles. So you almost have to complete side missions. It all sits together so seamlessly, and is so well integrated that you barely notice it. But Driver San Francisco compels you to play the whole game.
The story missions are similar to Driver of old: get from A to B in a given time, perform takedowns, tail baddies to their lair. But often you have to do these missions without the benefit of your mind-possession powers, and without tricks such as boost and ram.
