This is one of two lift bridges on Cherry Street that can raise to allow boats access to the inner areas of the Portlands.

Signs like these are there to warn the many cyclists, joggers, urban explorers and drivers who come into the Portlands of the potential dangers of crossing a lift bridge.
There is sometimes confusion as the Ship Channel Bridge, pictured here, and the Cherry Street Lift Bridge shown in the first photo of this gallery are both on Cherry Street. The Ship Channel Bridge is the more southern of the two, and is not far from Cherry Beach.

This is the view facing north however, back towards the Cherry Street Bridge and Lakeshore Boulevard.