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).
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 agree that the maximum safe traffic flux diminishes at higher speeds. Speed and traffic flux, however, are not independent variables. I would suggest that if traffic flux increases beyond the level that is sustainable at current speeds, but not beyond the optimum level, the increased flux will cause motorists to slow down. Motorists would not reduce the capacity of the road by traveling fast--rather, they would have their speed limited by road capacity.
It is certainly possible for people to reduce road capacity by traveling too slowly; further, for any non-trivial level of traffic flux there is a minimum safe average speed as well as a maximum. I'm not clear, however, how reducing the maximum allowable speed on a road would increase the traffic flux, except in cases where loading a right of way to full capacity would prevent other cars from entering.