Germany's Autobahn is famous around the world for having no mandatory speed limit in many sections. But while it's not uncommon to see flashy sports cars speeding by, it's not always free cruising.

There are in fact speed limits on some stretches of the motorway, and there can also be terrible traffic jams - especially during peak holiday seasons. At times like these, motorists may have to merge lanes and slow to a snail's pace.

This is where the Reissverschlusssystem, or "zipper system," comes into play. The zipper rule overrides the normal right-of-way rules when traffic is congested and lanes are merging. This orderly system means vehicles from each lane give way one at a time, thus resembling the interlocking teeth of a Reissverschluss, or zipper, being zipped up.

In the above photo from 1965, cars from Frankfurt merge behind a sign demonstrating the Reissverschlusssystem. It reads: "After you, please."