A while back, I posted this video to Youtube:
Earlier tonight, a user on Reddit asked how to do this very thing for a TV production he was working on. I obliged him by doing a quick write-up of the process; after which, I decided to share here as well:
First off, equipment used: Canon 60D with Sigma 17-200 lens (Magic Lantern installed -http://www.magiclantern.fm/), Tripod, FCPX
I used ML to shoot 216 photos using the intervalometer function. Since this was a "spur-of-the moment" idea I had, I didn't really think things through and didn't think to shoot any other format than full-size RAW. Looking back, I could've probably shot in the lowest res RAW or even JPEG and have been fine, since the resolution I was aiming for was 1080p. This would've let me shoot longer, but I quickly ran out of card space at that resolution. I squeezed off a few test shots at various shutter speeds until I found a speed that I liked the length of the trails at, settling on .8/second. (ISO 400)
Also, since I primarily shoot stills, I'm used to usually shooting in shutter-priority or aperture-priority and, since I'd already determined that I wanted a specific shutter speed (0.8 second), I made the mistake of shooting in shutter-priority. I should've shot in full manual, as this would've kept my aperture constant as well, preventing the variations in black levels that are evident in the final video. (The varying brightness of the cars' lights made the camera constantly adjust aperture--this probably could've been fixed in post, but I was just experimenting).
After getting back to my Mac, I downloaded the files to Lightroom and edited the first photo for noise reduction, sharpness, tone curve, exposure and color balance. I also cropped it to 16:9. I then synced this first image's edit to the rest of the images in the sequence. I exported out to a folder at 1920x1080, then loaded up FCPX.
In FCPX, I imported the photos, created a new project (1080P/24) and placed the photos onto the timeline. To actually create the timelapse, I followed the demo in this video: https://vimeo.com/38598208
Export and you're done.
Pretty easy, eh? Incidentally, I was just messing around with this video last night, experimenting with using Photoshop CS6 video features by applying various filters to the video. Here's a version with polar coordinates applied and some lengthening and reversing thrown in for good measure: