To remove people or moving cars from a picture in a single exposure, all you need is to increase the exposure time (shutter speed). That's not always easy with existing equipment. You may need to extend the shutter speed up to several minutes. But it could be achieved by using Neutral Density (ND) filters. The thing works because the amount of light reflected from people is so much smaller than from the environment (or building). So longer the exposure, the harder to notice the moving objects.