I love these!
And I still can't do them!
Made a sorry attempt one night in the city, on the way home from a Sens' game - thought the 30 miles of traffic would be ideal...
I just put the camera on a tripod, set it for a minute's exposure time, set the shutter for two second delay (the shutter trips two seconds after I press the button, negating camera shake and blurred recording), and stood back to watch the cars go by.
Are you taking about when you're driving (or, more safely, being a passenger) and you hold the camera for a long exposure, capturing the blurred lines of light along the street?