But it's not a roundabout. If you look carefully at the ends of the ring, you'll see that there's no way to turn around.
So that's not why the Laguna Garzon Bridge in Uruguay is shaped this way. This bridge is in the middle of a large bird habitat. Rafael Viñoly designed the ring to force drivers to slow down, causing less of a disturbance to the birds. Paul Keskeys writes for Architizer:
“The rotunda signals the terminus of the National Route 10,” said Viñoly. “I accepted to design the bridge on the condition that Route 10 — a federal highway that has been the main access to the beach resorts along the Uruguayan coast and one of the most destructive planning drivers of the territory — be downgraded to local jurisdiction in order to better control the development process."
Opening in December 2015, project investors have also praised the completion of the ring-shaped bridge. “It is an iconic architectural piece that will be a catalytic factor in driving the development of Rocha’s coastline,” said Argentine Eduardo Costantini, who invested 80 percent of the $10 million construction cost.
-via Lost at E Minor