Question: app to let someone know you are driving?
Answer: AT&T DriveMode. Availability: Android & iOS. This app was developed by AT&T but is available for both Android and iOS phones, regardless of the carrier. ... 
Down for the Count. Availability: iOS. ... 
LifeSaver. Availability: Android & iOS. ... 
Drivemode. Availability: Android. ... 
Drivemode Dash. Availability: iOS.