drivebysh00ter / Flickr

Stand clear of the closing doors, please.

Dear CityLab: Why do some subway doors open up when I try to squeeze through, while others keep closing?

Doors equipped with “sensitive edges” will reopen when an obstruction—like, say, your arm, leg, or bag—is detected. Then, the doors either close automatically, or the train conductor has to manually initiate the sequence.

Doors without sensitive edges don’t retract automatically. They keep closing against the pushback, maintaining pressure on whatever’s trapped between them—which is bad news if you’re the obstruction.

But even doors without sensitive edges can reopen under certain circumstances. For example, abnormally high electrical current draw from the door motor may trigger the retraction sequence as a safety precaution; the elevated activity alerts the system that a potential obstruction may need to be cleared. (No matter what, though, subway doors are designed to stay closed while the train is in motion. This is called “interlocking.”)

(Oran Viriyincy / Flickr)

To riders who habitually slip on board as the train doors are closing, sensitive edges might seem like a no-brainer. But Martin Schroeder, chief technology officer at the American Public Transport Association (APTA), says there are several good reasons for cities to think twice about installing them:

  1. Reliability: Trains are complicated systems, and every circuit, signal, and control is an opportunity for mechanical failure. “If you have additional sensing elements in the door, such that if it comes in contact with something it opens up again, that can be a reliability issue for all these [doors] that they have to put in,” Schroeder says.
  2. Safety: If the retraction mechanism fails, sensitive-edge doors might open while the train is in motion, putting lives at risk.
  3. Dwell time: Holding doors open can delay or disrupt train service, as more and more passengers attempt to board.

Individual transit agencies have to weigh these concerns against the increased flexibility afforded by sensitive edges.

Advances in rail technology make sensitive edges a safe option for many transit systems. But, whether the doors in your subway are sensitive or not, don’t tempt fate. “Stand clear” when you’re instructed to do so.

(Ted Sullivan / Flickr)

What happens if I do get my arm stuck in the door?  

It’ll hurt, but it shouldn’t do any serious damage. That’s because the Federal Railroad Administration (FRA) is working on some standards for door closure force right now. The proposed “Passenger Train Exterior Side Door Safety” rule, published March 2014, adopts the APTA standard for obstruction response. Train doors will not be able to close on you with more than 45 pounds of force. The regulation continues:

When an obstruction is detected, the door system shall react in a manner that will allow the obstruction to be released. A method for detecting an obstruction and preventing the closure of a powered door shall be included as part of the design of the door controls. The doors shall not close and latch to permit a closed-door indication if an obstruction is detected.

FRA’s rulemaking process began in 2007 after a number of incidents where train side door malfunctions resulted in passenger casualties—including the horrific case of a man who was caught in the doors of a New Jersey Transit train and dragged along the platform to his death. The agency called for new regulations to reduce the number and severity of door-caused injuries and improve passenger safety on the whole.

FRA is in the final review stage for this rule and expects to publish it “in the near future,” according to an agency spokesperson. In the meantime, don’t lose sleep over it. Door-caused injuries are extremely rare. According to 2011 data, only about 19 occur nationwide every year.

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