Jury Trial-Technical Matters

(Note: This is one part of a multi-part series of posts about a case I tried in June. It starts below.)

In order to understand some of the issues in the case, you need to have a basic understanding of how the Stanley automatic doors work. Each sliding door is equipped with two motion detectors and one presence sensor. The motion detectors, as their name implies, detect motion on both sides of the door. The presence sensor is situated on the top of the door, and detects the presence of mass within the door's threshold.

These particular doors are what is called "detector dependent." This means that, as you approach the door, the motion detector picks you up and opens the door. As you pass through the door, you are 'handed off' to the presence sensor. Then, when the motion detector on the other side of the door detects you, it turns off the presence sensor. Why? Because if it didn't, the door would continually reopen as the presence sensor sensed the door itself.

If you stop in the threshold of the door and stand there, the presence sensor is supposed to keep the door from closing on you.

In the case we tried, the door closed on our client as she stopped to allow the elderly gentleman to pass through. One week after the accident, someone from Albertson's called the service company and he came out and inspected the door. Initially, the door seemed to work fine. He noticed, though, that when he got down on his hands and knees (simulating a child), the door closed on him, indicating that the presence sensor in the door was not functioning properly. He replaced it.

Considering that the presence sensor was faulty, and given that the presence sensor is what, in theory, would have stopped the door from closing on our client as she stood in the threshold, we concluded that the faulty presence sensor was likely the cause of the accident.

So what?

Albertson's defended the case by saying that they, in effect, performed daily safety checks all day every day. In other words, people used these doors all the time. If there was a problem with a door, they fixed it.

Our contention was that simply letting your customers walk through the doors was not enough. The other part of the daily safety check requires that someone stop in the doorway to check the presence sensor. By failing to do this part of the check, Albertson's was unable to detect the problem with the presence sensor.

But no, Albertson's countered. Mechanical failures are asymmetric events. In other words, they could check the door three times a day every day, and the door could still fail immediately after it was last checked (which is true). Further, when the serviceman found the faulty sensor, he had to get down on his hands and knees, something that is not required by the daily safety checks.

Wait a minute, though. Just because all mechanical failures are not always detectable is not an excuse never to test for them. Do you ever check your tires? Do you have your furnace inspected? We argued that, what Albertson's did was use its customers as their "canaries in a coalmine," to detect problems with this dangerous machinery on their premises.


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