Route difficulty: ⭐️⭐️⭐️ [run] This is a popular local route around Lady Bird Lake via the pedestrian walkway under the Mopac and Bat Bridge (Congress Avenue Bridge) Begin at the Fairmont Austin, run along the Colorado River. Cross the river via the pedestrian walkway underneath the Mopac Expressway, run along the other side of the river and return via the Congress Avenue Bridge - home to the world's largest urban bat colony. Between 750,000 to 1.5 million bats reside underneath the bridge each summer. They emerge from underneath the bridge at dusk in the Summer.