Like if one half of a ship needs to get past?

CHICAGO -- The city began restoring sidewalks and handrails on the historic downtown Michigan Avenue bridge today -- the first step in a multistage project that will stretch until June, disrupt pedestrian traffic and cost in the neighborhood of $3.5 million....

The work is being done one quadrant at a time to minimize pedestrian snarls and to leave one half of the bridge available to be raised in case of emergency...

[From Michigan Avenue bridge makeover begins ]