You have probably heard that passengers on long flights may develop dangerous blood clots. What you may not realize is that people who take day trips in cars or trains may also be at risk. A recent World Health Organization report clarifies that any form of travel longer than four hours increases one's chances for deep vein thrombosis (DVT). Blood clots in the legs can be deadly when they break away and travel to the lungs, heart, or brain. To counteract periods of immobility while traveling, plan to take hourly walks and do exercises to contract your calf muscles. Avoiding sedatives and alcohol is important, too. Treatment is most effective when DVT is detected early. Symptoms include calf pain or cramps and swelling of the leg.