red zone 
noun
1A red sector on a gauge or dial corresponding to conditions that exceed safety limits: ozone readings edged into the red zone...
- We are somewhat dubious that the risks posed to the banks are in fact ‘a long way from the red zone of the dial’.
- When I find the dial, I'm gonna crank it up to the red zone.
- So while eyeing his gas gauge, which dips precariously into the red zone, Coffman winds in and out of a mind-boggling number of developments until he reaches one called Northridge.
1.1A region that is dangerous or forbidden, or in which a particular activity is prohibited: any officer who parks in the red zone outside the courthouse could receive a ticket...
- The Cherokee was illegally parked in the red zone.
- He said people living within the red zones - areas that have been prone to flooding and landslides during recent inclement weather - would be prewarned.
- The areas have been divided into zones and people in the red zones have been advised to vacate these areas in case of severe weather.
1.2 American Football The region between the opposing team’s 20-yard line and goal line, which is a major focus of their attack strategy.Second, let's talk about what these teams do in the red zone....
- It was the first time in the game that either team had entered the red zone.
- Lewis could help the team in the red zone, where he is a big, safe target.