How to avoid blur and overexposure in urban long-exposure light-trail shots?

[Kyle36]

I've been experimenting with long-exposure photography to capture cityscapes at night, but I'm consistently getting results that are either too blurry from camera shake or overexposed despite using a narrow aperture and low ISO. I'm using a sturdy tripod and a remote shutter release, so I'm puzzled. For photographers who specialize in this, what are the most common pitfalls with long-exposure settings in urban environments, and how do you perfectly calculate exposure time for moving light trails?