I often see after a crash that people die unable to exit the aircraft. The news reports and passenger and safety organizations criticize the FAA for not having realistic emergency exit regulations. What determines the criteria for a plane being "safe" in an emergency?
Per 14 CFR 25.803:
For airplanes having a seating capacity of more than 44 passengers, it must be shown that the maximum seating capacity, including the number of crewmembers required by the operating rules for which certification is requested, can be evacuated from the airplane to the ground under simulated emergency conditions within 90 seconds
Appendix J to Part 25 specifies exactly how the evacuation tests should be done. It's too long to quote here, but it includes requirements for lighting levels, number of passengers, minor obstacles, infants on board etc.
I have no idea whether or not those simulated conditions are reasonable and effective, but they at least set a standard.