I really love shooting with long exposures. It brings out really amazing effects.If you have moving vehicles then they leave behind those magical trails of lights.
The exposure depends upon the amount of time the shutter is open. To capture those streaks of light a shutter time of 1-4 seconds is enough depending upon the lighting conditions.

Here are some helpful tips to capture those trails of lights:

• Choose a broad street with busy traffic.
• If possible use a tripod.
• If you do not have a tripod then rest your camera on some support so that it doesn’t not move while taking the picture. you will have a nightmare time if you try to use just your bare hands unless you have a very steady hand and i mean very steady.
• While pressing the shutter the camera may move a little leading to blurry images, so if possible use a self timer or a shutter release cable.
• Use manual mode or shutter priority and set shutter speed of a few seconds depending upon the lighting conditions and the length of the trail that you want. Remember longer the shutter speed longer the trail of light.
• Use narrow aperture for good Depth of field and to achieve slower shutter speed.
• Use a low ISO setting of about 50 or 100.
• Experiment. Try shooting from different angles.

Here are some pictures that i took..