It may seem too little and too late, but U.S. and global central banks are admitting that the economy is at risk and that the summer's credit markets crunch may have broader ripple effects. Behind cheers over new highs for many indices, the stock market has been reflecting this negative sentiment. Largely considered a proxy for U.S. economic growth, the Dow Jones Transportation Average has failed to make new highs with the rest of the market. It peaked with the others on July 19, and bottomed with the broad market in mid-August when the credit markets seized up. But, instead of making new highs like its brethren the Dow Jones Industrial Average and the S&P 500, the Transports remain 11% below their July peak. Executives at several of its component companies like FedEx ( FDX), Ryder Systems ( R) or trucking company YRC Worldwide ( YRCW) have lamented declining shipping volumes and U.S. business strength as homebuilding trails off and imports fall. Another proxy for strong economic growth, the small-cap Russell 2000 index, which has been a leader over the past four years, has likewise failed to make new highs in the post-August rally. It came close at the end of last week, but remains 3% below its July high. The markets' anxiety about economic growth was echoed by central banks Tuesday, though some say their warnings are behind the curve.