The Transportation Security Administration is trying to speed up airport screening by asking passengers to choose a line based on their familiarity with checkpoint procedures. But human nature being what it is, this approach may hit its own snags: people typically opt for the shortest line, and all think they are experts.

“In theory, it’s a good idea. It lets people say, ‘This is my comfort level,’ ” said Steven Frischling, a photographer from Connecticut who encountered the new system in Salt Lake City and Boston. “The problem is, when people show up, everyone thinks they know how to get through security.”

The agency is using a ski slope metaphor for its three new “self-select” security lines.

The system, which has been introduced at 21 airports and is coming soon to New York, has a black diamond line for expert travelers, defined as those who fly more than twice a month and are skilled at security procedures, always ready with items removed; a blue square for casual travelers, who are familiar with the screening process; and a green circle for families and those needing assistance or more time.

The idea is that passengers will select appropriate lines for their situations. The reality has been mixed, according to travelers who have used the lines, which were introduced in February in Salt Lake City and Denver.