They're everywhere. Connecting roads large and small, organising and smoothing the conflicting needs of traffic. Helping people get on and off busy motorways, providing a pleasant transition from open road to suburb, making the flow of urban streets more efficient. It's amazing how productive it can be to go in circles.
The humble roundabout has been with us for a century now, in various forms, through all manner of fashions for straight sides and sharp corners, different systems of priority, one way and two way traffic flows.
These days, traffic lights are probably multiplying at a faster rate, and "give way" junctions are far more numerous, but roundabouts are the ones that have suited us best and become part of our culture. To people overseas, they have often become associated with all things British.
So, to mark the centenary, here is a celebration of all things circular - and please, don't forget to indicate.
Where else to begin but a pilgrimage through London, Paris and New York to the first British roundabout, Sollershott Circus.
They might look nice, but roundabouts don't really work until you establish rules to bring order to the chaos.
A large part of the twentieth century was spent perfecting the art of the roundabout, and for a long time a circle wasn't the favoured shape.
They don't like to discuss it, but within the heart of every British citizen is an inexplicable fondness for roundabouts. Why?
Mini-roundabouts, small roundabouts, big roundabouts, spirals, signals, hamburgers, hot cross buns. Roundabouts have come a long way - what's next?
Research sources for this article are listed at CBRD ArchiveWiki.