So everyone I know uses their smartphone while driving. When stopped at a stoplight, they text, surf the internet, etc. Generally, they don't know when the light changes. There should be an app for that. The display would show you the color of the light you're sitting at in the corner of your tiny screen, and perhaps buzz when it changes.
barcelona: helpful, common, worth about a dollar
real madrid: i have no idea if traffic light data is public, and if so, who and how you control it