Perhaps one way to illustrate it is using an extreme--imagine if cars were going 550 mph. You'd see few cars on the road with such distances required between each. If you were one of the lucky ones, on the road, your trip would be short...but most people would be sitting in traffic. If the speed limit on an expressway with a metered entrance were set at 550mph, and if the goal was to keep traffic on the expressway moving at top speed, the allowable rate of entry would indeed have to be slower than if the speed limit were lower. On the other hand, in more practical scenarios, vehicle speeds will be determined by traffic density rather than vice versa. I'm not quite clear how the actual speed of travel on a road would exceed the optimum speed under such a scenario.
You're misunderstanding the point I'm making.
When traffic is faster, the braking distance is greater--and not in a linear manner. Therefore, if safe distances are to be maintained, you have to have LOTS more space between vehicles in fast traffic than in slow. Therefore, you get FEWER cars passing a point for every minute when cars are going fast than you do when they are going more slowly.
I know it sounds paradoxical, but look at the center (oversimplified) diagram...
or this PowerPoint presentation...
More can be found in Chapter 2 (2.3.2).