Transportation stocks have been stuck in reverse.
The Dow Jones transportation average has fallen 19 percent in 2015, widely underperforming the Dow and S&P 500, which are down 3 and 2 percent respectively year to date. According to some market participants, the dismal returns could be flashing a warning sign to the overall market.
"This [underperformance] really brings in the concern about the Dow Theory," Craig Johnson said Friday on CNBC's "Trading Nation." Johnson, of course referring to the age-old technical indicator that suggests that steep declines in the transportation index will eventually spill into the broad market. "The industrial stocks haven't been working, and the transports aren't working, either," he added. "That puts a bit of a question mark on the overall market at this point in time."
The transports, which is composed of airline stocks, railroads and shippers, is the second-worst performing industry group behind energy this year. All but one of the stocks in the index are negative in the last month.