According to the New York Times, the travel alert for Europe by the U.S. State Department has not made travelers more alert, just more confused. The vagueness of the warning, combined with its broad definitions of places to avoid, has left travelers unsure of how to proceed with vigilance. While not specifically advising against travel to Europe, the State Department cautions that "U.S. citizens are reminded of the potential for terrorists to attack public transportation systems and other tourist infrastructure. Terrorists have targeted and attacked subway and rail systems, as well as aviation and maritime services." As it is impossible to travel without making use of transportation system infrastructures, particularly airports, many tourists remain bewildered as to how to heed the warning. Guidelines provided by the State Department such as "U.S. citizens should...adopt appropriate safety measures to protect themselves when traveling" without offering specifics, provide little help for travelers.
State Department Advisory: http://travel.state.gov/travel/cis_pa_tw/pa/pa_5171.html
New York Times Article: http://www.nytimes.com/2010/10/04/world/europe/04security.html?ref=travel