https://phys.org/news/2018-07-thousands-purple-birds-swarm-outer.html

Each year, thousands of shiny songbirds—purple martins—choose to roost under the bridge, creating a potential hazard for both birds and people.

The NCDOT has established a special 20 mph speed limit zone and set up flashing lights at the bridge to protect the more than 100,000 birds that roost under its west end, especially since the birds tend to commute at the same time people do—in the morning and evening. The bridge speed limit is normally 55 mph.