I actually just shot a photo just like this. This was shot around mid afternoon in downtown Portland and I assure you, there actually was traffic moving along the bridge.

The set up included stacking an 18 stop ND filter on top of a polarizer and exposing the frame for 11 minutes, ish (707 seconds to be exact). As long as the people or cars keep moving, at that slow an exposure, they won't be there at all, not even as partially exposed ghosts.

So, the technique is...

• Get a massive ND filter or filters (it would be good to do some mock calculations to see what you need)
• Lightproof your camera - I used a few layers of painter's tape over the side seals and over the viewfinder. I also used a Canon soft lens bag with the bottom cut off to sleeve over the lens
• Use a tripod and shutter release (I forgot mine for this shot and literally held the button down the whole time. Not fun.)

Using this technique, there will be a very high amount of color noise, especially noticeable in the shadows. I highly recommend taking a dark frame.