I often walk through my quiet neighborhood streets on weekday evenings or weekend mornings.  These are the times when I don’t expect much traffic, and my expectations are usually met.  The probability of a dangerous event that would injure me seems pretty low (risk assessment), since I am usually on the sidewalk and any cars are in the street.  However when I come to an intersection, I am constantly amazed how often one of the few moving cars on the street reaches the intersection at the same time as I do (demand condition).  Thank goodness for stop signs and low speed limits (layers of protection relying on operator, but no automatic safety function).  But what if a driver does not see the stop sign in time, or they don’t see me (operator fatigue or distraction)?  This can raise the likelihood of an accident.  So I also must be vigilant and look for any conditions that pose a risk to my safety.  When all cars are equipped with collision avoidance systems (automatic safety function), especially systems that can detect pedestrians, many of these risks will be mitigated and the risk to my safety will decrease.

A winter storm occurred in my neighborhood earlier this year.  The utility wires, which hung across the same uncongested neighborhood street where I walk, became coated with ice.  The wires got heavy, pulled away from their support (random fault, stress exceeds strength), and fell to the ground.  It just so happened that my car was traveling under those wires at the exact time they decided to come loose.  It seems like the odds would be pretty low for that combination of events but, after having to replace my windshield, it seems like those wires were just waiting for me and my car.  Unfortunately, the wires had no stop sign to obey.  If only I could have invoked the Startrek-like command “Shields up, Mr. Chekov!!” when I left my driveway.

Independent events like this happen all the time.  As slim as the odds may seem, sometimes they do occur simultaneously and cause accidents.  You’re fooling yourself if you think a single mitigation factor can eliminate all your risk.  You need to consider combinations of likely events that could reduce the effectiveness of your layers of protection.