One study that tracked drivers in Boston during rush hour found that if you could remove just 1 percent of people on the road (say, to mass transit), you could

One study that tracked drivers in Boston during rush hour found that if you could remove just 1 percent of people on the road (say, to mass transit), you could achieve a whopping 18 percent improvement in traffic flow. But for whatever reason, cities haven't figured out how to do that just yet. If you don't think in static terms, this is obvious. Across any segment of time, some fraction of commuters will change their habits based on their expectations of time/cost/comfort. Some commuters can respond daily if, for example, they have a car and affordable parking at their destination as well as access to public transit. Other commuters may not have a car at all but will eventually buy one if it begins to offer substantial advantages over its cost. The point is that the system is dynamic. And if, magically, the City of Boston could wave a wand and redirect 1% of its commuters to the subway, they'd soon discover that the 18% improvement in traffic flow has tempted many other commuters back onto the roads, and the benefit vanishes.