Whether our flight’s about to head down the runway or not, we all know that sinking feeling of turning an airport corner and running smack into a snaking crowd of passengers waiting to pass through a TSA checkpoint.

Happily for passengers, a real-time tool in place since August 2012 at Dulles International Airport will tell you how long the lines are at any given time.

The system, designed by Blue Eye Video, took advantage of security cameras already in place, and the airport added a few more, said Todd Sheller, the Metropolitan Washington Airports Authority’s enterprise business innovation manager.

He said that the wait-time system works by putting together a picture of the line and counting how many people are in it. It tracks the number of people joining and leaving the queue. “We know then what the consumption is,” Sheller said.

The information is displayed on monitors at the airport’s departures level and is also available on its desktop and mobile Web sites.

The idea is that you can use the information to decide when to leave for the airport and which of the two TSA checkpoints — east or west — to head for once you arrive. Both checkpoints take you to the AeroTrain that you hop onto to get to your gate. (The system doesn’t currently monitor the TSA PreCheck line.)

Jordan Thaeler, co-founder of WhatsBusy, an algorithm-based app that gives airport wait-time estimates, said that passengers at Dulles tend to favor the west checkpoint. But lines at each station can vary depending on the time of day and the flight schedule.

Dulles is the first airport in the country to have the Blue Eye video system in place. According to the company, several other American and Canadian airports are interested in or in the process of testing its technology. The airports authority has begun to look into implementing the video system at National, which may take extra effort because of the airport’s four checkpoints.

Thaeler said that other airports have experimented with a system that picks up on Bluetooth signals to track people moving through security, but it has been problematic, because most people don’t enable the Bluetooth function on their devices. Also, the system isn’t precise enough to indicate whether a person attached to the Bluetooth signal is actually in the line or just near it.

Sheller said that Dulles spent about six months with the video system in pilot mode before it went public. Estimates generated by the cameras were compared with those obtained by TSA, which also monitors wait times. The agency randomly selects people entering the security line, hands them a card with the current time on it and then gets the card back as passengers enter the checkpoint to calculate how long they stood in line. The difference between the Blue Eye and TSA numbers was about two to three minutes, close enough to inspire confidence that the system was accurate.

However, there will always be variables, whether it’s a giant tour group of schoolchildren or a checkpoint lane opening or closing. So use estimate tools to make an informed decision, but don’t stake your vacation on them.