There's no such thing as demand being at 100%, certainly not in this context. Demand is a curve. If the cost of driving decreases--i.e. less congestion, easer traveling--people will drive further and more often. It won't happen instantaneously, of course, but demand will adjust even if the population stays the same.
That doesn't mean the new road will fill up completely. There may be other constraining factors. But if a route was highly congested before expansion, you can bet demand will go up after expansion.
> "A year after the freeway opened, traffic volumes along parallel roads like 44th, 56th and 68th streets dropped 40–50%"
That doesn't tell you anything about the total volume of traffic. And in any event it's precisely what you'd expect to see. If a line at the checkout counter has 10 people in it and new counter opens up, expect about 5 people to immediately move over. But _also_ expect a few more people to join one or the other line now that they'll clear quicker.
That doesn't mean the new road will fill up completely. There may be other constraining factors. But if a route was highly congested before expansion, you can bet demand will go up after expansion.
That doesn't tell you anything about the total volume of traffic. And in any event it's precisely what you'd expect to see. If a line at the checkout counter has 10 people in it and new counter opens up, expect about 5 people to immediately move over. But _also_ expect a few more people to join one or the other line now that they'll clear quicker.