The site won't let you link directly to a photo, it's a "rotator", so you always see a different image.

If you have a very fast camera taking rapid fire photos, the compositing would actually be pretty easy, just tedious as hell (especially the freeways, wow). I find it telling that nearly all of the photos seem have been taken on overcast days, which rids the image of annoying shadows. There's one with blue sky but late in the day so the freeway part is mostly shadowed anyway. Smart and nicely done.

Actually, the new CS5 Photoshop has a nifty new Content Aware Compositing feature which now makes this a breeze.

what intrigues me is that I still sense the motion of the freeways like they are full of cars moving too fast to be seen.

This reminds me of a recent time-lapse film I saw, called "Running on Empty." Definitely worth a look!

http://rossching.com/running-on-empty/