With oil prices and interest rates low, more and more Americans are buying boats and as traffic on the waterways increases, so does the number of accidents from drinking and driving on water．A National Transportation Safety Board study concludes that drinking may be a factor in 80 percent of the 1 000 or so deaths in boating accidents that occur every year．Yet anyone can drive a boat without a license(执照), drinking on board is almost universally legal．Experts say drunken boating may be even more erous than drunken driving．Boating on a crowded waterway can be in complete disorder: there are no stop signs, traffic lights or lane markers．Speed restrictions are nonexistent outside harbors．Some boats have terrible power and top speeds and no boat has brakes(制动器)．Combined with the effects of all the sun, wind, waves and happy tiredness that go with boating, even moderate amounts of alcohol(酒精) can be erous．According to one recent study, a boater's judgment and behavior．can be worsened by only about a third as much alcohol as it would take on land．Only recently have many states begun to take action against drunken boating．The key is passing laws to set a medical standard for blood alcohol level and to allow police to test blood-alcohol levels on the spot．Increasingly, speed restrictions are also being set on crowded waterways．Some experts also suggest granting licenses for boat operators．The license requirement could be used to strengthen safety training and to keep repeat offenders off the water．It would also make it harder for beginners and children to go joyriding in high-performance boats.
What is the main cause of the increase of traffic on water?