It's a neat thought, and everyone who's watched a light turn red ahead of them just as they cleared a green has thought it before, but there is a serious downside. Urban stoplights should be optimized for the safety of non-motorists (pedestrians and cyclists) above all else. Giving cars more uninterrupted travel leads to higher speeds and more collisions. I'm happy that where I live there's more attention paid to traffic calming and bicycle infrastructure than motorist travel-time optimization.