When I lived there up to a couple of years ago, they were surprisingly accurate, even during fairly heavy traffic. I can't recall the exact details, but from m

When I lived there up to a couple of years ago, they were surprisingly accurate, even during fairly heavy traffic. I can't recall the exact details, but from memory they use road counters at entries and exits to measure traffic numbers within sectors and, based on theoretical maximum number of cars per sector, predict whether traffic is capable of flowing at the speed limit or at some calculated reduced speed for each sector. The speed calculations, along with each sectors' distance, give you predicted trip time to the various exits on the display. As I said, don't quote me, but that was my understanding.