One of the main things that I wanted to capture the other night when photographing from Skypoint was the car trails as they move throughout the city. Car trails really make the city come alive and add some energy to the scene. To capture these I took a series of multiple longer exposures between 5-15 seconds without changing the position of the camera. Then in Photoshop I loaded all of the files into layers. I selected all layers, set the blend mode to lighten. This allows the lighter part (car trails) of the underneath photos to show through. If it causes other areas of the photo to be too light you can add a mask and paint out the too light areas or inverse the mask and just paint through the car trails.