Golden Gate Bridge
Can you believe it, the Golden Gate Bridge, no clouds, no fog and what, NO CARS. Not exactly correct, approximately 120,000 vehicles cross the Golden Gate Bridge each day. But, use a 2-8 stop Vari-ND filter from Singh-Ray Filters www.singh-ray.com/varind.html for an 8 second exposure and the cars just disappear. I might add that too long of exposure will not give a greater invisibility of vehicles but instead may introduce so much blur as to soften the image within the areas of vehicle travel.