To alert others with your voice, you must shout loudly if you are to be heard from 30 feet away. But this can be difficult, especially if you are winded from your run. Moreover, shouting can startle pedestrians, which can be uncomfortable for everyone.
Instead, use Runbell. It provides a clear, bright, and polite ding to alert others to your presence from 30 feet out, which gives pedestrians time to react.