This picture looks like it was captured shortly before sunrise. This would allow Porter to capture the image with a very long shutter speed without washing out the image with exposure. The picture is also taken at an interesting angle giving off somewhat of a tilt-shift effect.
There are a few ways to avoid too much exposure when shooting with long shutter speeds. The first is to use a smaller aperture. This will cause less light to enter the camera. The second is to use a lower ISO. The last method is to use a neutral density filter (or shoot at night).