That is the maximum number of cars that can pass a given point in 2 seconds; it does not mean that a road operating at peak capacity can reach that number.

You also have to take into account cars merging onto the road you are on. Imagine a road acting at peak efficiency, with a car passing every 2 seconds. Now merge in more cars. They are going to push back every car behind by that same 2 seconds.

You can read the section on 'merging' in the link above to read more details.