Bullhorns
Bullhorns don't work very well at races.
Noise from runners and from the surrounding area interferes with the sound emanating from a bullhorn. Runners only a few rows back from the start line usually can't hear anything said through a bullhorn.
To effectively communicate with runners, rent a sound system (if you produce lots of races, it may be more economical to purchase one). Then, your words will be clearly heard by everyone.
Instructions, warnings, announcements, sponsor pitches, and anything else you need to say must be amplified clearly if anybody is to hear them. If the start and finish lines are in approximately the same location, you can use the same sound system for both.
A sound system is not that expensive and it's worth the money.