Reading about urban planning it often sounds like the engineers back in the 60s had no clue. They thought it was just great to bulldoze whole neighborhoods to build freeways partly because freeways made people drive faster, thereby reducing the amount of pollution they emit because cars are more efficient at higher speeds. But really it just encourages people to live farther away and drive more, increasing the amount of pollution (and traffic, and sprawl).
However, we still don’t know everything today. This Salon article (watching an ad required if you aren’t a subscriber) provocatively suggests that reducing devices like traffic lights and painted road lines in cities, stopping trying to keep pedestrians and traffic apart, actually increases both safety and speed. Safety because it forces people to drive slower and use their brains more, and speed because driving continuously around 20 miles an hour isn’t worse than sitting at an interminable chain of traffic lights.
That’s pretty radical, and I’d like to see more research to back this up before really believing it. But this is the kind of thing we need to experiment with to understand. Europe and especially Scandinavia, always ahead of the US in this kind of thing, is starting to. But it will take a long time for the US to get there.
We still have most state and local Departments of Transportation focused on getting cars to move faster, who still react to congestion by designing a new and larger freeway ramp with more separated movements. It’s easy for engineers to believe that faster is more efficient - after all, it’s true for most systems. And that’s easy for voters to understand. Traffic just always manages to defy common sense expectations.