Metro has had 5 percent of its 588 escalators out of service this week. But what does that mean for riders?

A reader pointed out that the likelihood of a rider encountering a downed escalator each day is much higher than 5 percent -- or one out of every 20 escalators. Instead it's more likely to be a one in three chance the rider will encounter an out-of-service escalator each day.

Take for example, a rider who takes Metro to and from work each day. That commuter likely takes eight escalator trips, if not more, in a single day. There's the trip from the street to the mezzanine, the mezanine to the platform on the way to the train and the same on the way out of the system. A return trip home doubles the commuter's exposure, reaching at least eight escalators. That gives the rider a one in three chance of seeing a down escalator on their trip.

The rate may effectively be even higher if one considers that some busy stations attract many more riders than others. If an escalator is down at Union Station, the busiest station in the system, more riders are likely to experience it than, say at Morgan Boulevard. Additionally, the most heavily used escalators may also be more likely to break down or need repairs due to extensive use.

So riders aren't imagining that they are hiking lots of escalators, even if the official number of escalator outages is low.