Traffic Trails
 

How do I put bright lines on a photo like the ones in this photo? Sorry for the stupid question :L

Attachment 6331

1 Find yourself a busy road/motorway with a bridge/flyover on a dark evening/night. Cars must, for obvious reasons, have their lights on.
2 Mount your camera on a tripod.
3 Set the aperture to a large f-number.
4 Set speed for a long exposure.
5 If the results aren't what you wanted - adjust 3 & 4.

I've changed the thread title as it has nothing to do with using a Canon 400D, Steve's solution works for any make/model.

As Steve said.
I would also add low ISO 100 ish. Shoot in RAW as will give more controle. JPEG will show more noise which can/will be a problem.
Have fun.