Generally cars are going to be faster (it's point-to-point with no stops to let people on/off), or equivalently, they have greater range for the same time. If your goal is to physically remove yourself from downtown, they let you go a bit further. Also you don't have to deal with someone leaving their mac-and-cheese meal to stink up the bus. Getting away from such unpleasantries is kind of the point.
Once you take in to account traffic, transit can definitely be faster than cars. A subway that bypasses traffic entirely, or even a bus using HOV lanes, can easily outpace a car, especially if it's an express bus (fewer stops) and services are centrally located.
I've worked at places where it could take 20 minutes to get out of the parking garage when a bus stop was less than a 5 minute walk, on-boarding/off-boarding was super fast (a pre-pay kiosk meant people didn't stop to pay when they got on), and I could be all the way home in less than 20 minutes.
