It was interesting to hear your thoughts on this topic, IMHO this is where your argument falls apart:
The engineers will increase the speed limit to increase the volume of cars, which operate in the given area.
Faster speed limits are to improve commute time at the cost of capacity and safety. There are lots of ways to look at the problem of how to accommodate lots traffic but when talking about the speed of the roadway no mater what the speed of the roadway is you will have a car/lane passing a given point ~every two seconds. For me the issue is how to accommodate dense travel in dense development. That is to say a mile of 60mph roadway can accommodate almost 28 cars/mile/lane at any given time. A 30mph road can accommodate 51 cars/mile/lane at any given time. So its the slower roads that have a higher capacity.
Part 2 of this problem is congestion is relatively safer then non-congestion. Typically we try to fix congestion with faster speed roadways rather then trying to smooth out the flow resulting in more accidents per mile of roadway.
The issue for me is there is too much focus on the individual and their car user rather then the system as a whole. Congestion means that dense forms of transportation is called for which translates to slower roadway speeds for motorists along with more mass transit and more biking and walking.