Automatic Traffic Lights in JavaScript with Different Intervals

I am doing a GCSE project about traffic lights in JavaScript. My automatic traffic light system already works fine, but I can only make the lights change with the same interval in between. I would like to make it so that the sequence stays on the green and red light for a longer period of time than it does on the Red-Amber and Amber. Any ideas?? I'm using Adobe Dreamweaver if that's important.