Evoke a busy night-time city with this clever effect
Images that use a slow shutter speed tend to lack sharp detail. Use a slight Gaussian Blur to soften the details. Filter>Blur>Gaussian Blur with a setting of 3px should work nicely.
Grab the pen tool, set it to Path mode in the Options bar (not the default Shape mode) and draw out paths to trace the motion of the vehicle lights. To create multiple paths, just Ctrl/Cmd-click to end one path so you can start another.
Grab the Brush tool and open the Brushes palette. Use the Soft Round preset with a Width of 9. Enable the Shape Dynamics and set the Size Control to Pen Pressure. Create a new layer and switch to the Paths palette. With the dialog open, Ctrl/right-click on the path and select Stroke Path. Check the Simulate Pressure box and hit OK.
Add an Outer Glow layer style set to a red colour for taillights or a pale yellow for headlights, Use the Eraser tool to remove parts of the streaks that should be hidden behind foreground objects. For additional streaks, edit the path points and stroke on a new layer.