Just as traffic circles are a lovely low-tech replacement for traffic lights and start and stop traffic, le disque bleue is a simple way to control municipal parking. Instead of big computerized boxes and paper tickets, it works like this in many smaller towns:
Parking spaces with white lines are just open. Park as long as you wish.
Parking space with blue lines are time limited, usually 90 minutes.
When you park you take your 5” square disque bleue, spin the disc so the arrow points to the time that you parked, then leave it on your dash board.
Traffic police can see when you parked, and can tell if you’ve been there too long.
So simple.