This picture here I made last spring from one of my favorite spots in Boston.
This time I used a little longer exposure. How long? (Hint: You may guess on the picture below that has the same exposure time).
Yes, you are right: 3 minutes. How can you shot 3 minutes or longer during the day? What you need is a very strong neutral density filter (e.g. ND10). But this is also pretty expensive. A year ago I came across this picture by Peter Carr and a pretty cheap solution.
If you want to filter a lot of light, why not using a welding glass as a filter (cost 2-3$). And yes, this really works. All what you have to care is a heavy white balance afterwards, because most of the time the picture are way too green. Shooting raw is therefore highly recommend.
The only lasting problem is to attach the glass to your lens. I used super glue to attach it to a filter ring, so I can screw it on my camera lens.Here is a photo of my setup.
Tell me how you like the results and the setup
All the best from Boston
P.S.: By the way here is the Black and White version (4B5WK94SKEAR)