Police are not allowed to enter the armed robbery area until it has been in progress for 120 seconds.
Many cops perform airstrikes (I'm one of them), and as you know, if you flew above the AR before the anti-rush system duration passes, you will die. So making the system linked to the danger level means that the duration will be changed every second, and it won't be stable, which will be confusing for airmen and even for ground cops.
Actually, you brought up something to my mind which I did not think of. Maybe we can take the last danger level value update before the AR begins, so the anti rush system will be dependant on that value.