I don't know much about cars or shooting at them, but...if you shoot the driver, doesn't the car then go out of control? Again, I don't really know anything about it, but it seems to me that controlling the car might be the first course of action.
A gun is a rotten tool for stopping a car, and the police have much better tools for doing so (like those tire rippers that they pull across the road in high-speed chases). But when a gun is the only tool at hand the best way to utilize it is to use it to neutralize whoever is controlling the vehicle. Short of an incredibly risky and foolhardy attempt to gain access to a moving vehicle, this cop had no other effective options.