Bridge of Mitheithel

The three-arched stone bridge that crossed the River Mitheithel, or Hoarwell, far in the east of Eriador. It was the Last Bridge on the East Road before the road reached the Misty Mountains, and the only means of crossing the river between its sources in the cold north and the broken bridge of Tharbad far to the south.

The Bridge's strategic position was important in Frodo's journey to Rivendell. As the only means of crossing the river, the Nazgûl hoped to intercept him here, and three of the Nine Riders lay in wait for the Hobbits. If they had not been driven away by Glorfindel, Frodo's quest would have ended at the Bridge of Mitheithel.

We can be sure that Frodo's uncle Bilbo also used the Bridge on his journey to the Lonely Mountain, years before. Though it's not identified by name, there's a mention of an 'ancient stone bridge' in chapter 2 of The Hobbit, and the location of that bridge seems to match well with the Bridge of Mitheithel.