Here’s a cool graph from the Puget Sound Regional Council that illustrates the “sweet-spot” for highway speeds. Apparently, traffic throughput is maximized at about 1,800-2,000 cars per highway lane (the horizontal axis) when vehicles are moving somewhere between 40 and 50 miles per hour (the vertical axis).
As the graph shows, when speeds are lower than that, or higher than that, then highways aren’t operating as efficiently as they might.
So it would seem (to me at least) that a key ingredient in reducing demand for new highways is to keep traffic on existing roads flowing at somewhere between 40 and 50 miles an hour, even at times of peak demand. How to do that? Metered on-ramps help; so would tolling the most congested highways.