"...lessen the need for being dependent on cars. we have limited space and can't be building more roads." -- aiea7
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Which is correct if and only if we are still in the 20th Century.  But we are not.  We are in the 17th year of the 21st Century
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Now then.  
This is what Pittsburgh Mayor Luke Ravenstahl said concerning a pilot project in Pittsburg: 
--"a breakthrough in making our city's traffic system work far more efficiently without having to resort to expensive widening roads, eliminating street parking, or re-routing".. resulting in ""reductions of 40 percent in vehicle wait time, nearly 26 percent in travel time and 21 percent in projected vehicle emissions reduction.
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What Pittsburg did in 2009 is adopt a new technology called "adaptive traffic signal control technology" for a network of traffic lights serving the busy East Liberty area of the city.  
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That technology is now widely used in mainland USA.