A D.C. proposal to install sirens across the National Mall to warn visitors of impending terrorism or other hazards has run into a federal roadblock over aesthetics, District officials say.

The National Capital Planning Commission objected to the pilot siren project based “on the look,” said Jo’Ellen Countee, spokeswoman for the D.C. Homeland Security and Emergency Management Agency. The protests, she said, were raised during a meeting several months ago and have yet to be resolved.