Have you guys seen this thread - https://www.waze.com/forum/viewtopic.ph ... 88#p834488
Basically, Waze does not alert for police or red light cameras if the phone is locked (e.g. when using voice navigation in my car with the screen turned off). Other people have the same problem, so using waze with the screen off is a very legitimate use case.
Sometimes, if I unlock the screen on my iPhone 5s right when I'm approaching police, Waze would alert on screen unlock - but without unlocking, it doesn't alert.