For safer streets, less congestion, and cleaner air, the bicycle is poised to become an integral part of urban transportation systems for the 21st century, says the Worldwatch Institute in a new report.

Too often relegated to weekend jaunts and children's use, bicycles are emerging as a solution to some of today's most intractable urban problems.

Putting bicycles to work could produce enormous savings, like reduced air and noise pollution, better land use, less congestion, and lower health costs. "Americans drive cars and taxis more than 1.5 trillion miles each year," said Gary Gardner, author of "When Cities Take Bicycles Seriously," an article in the September/October issue of World Watch magazine. "Shifting just 5 percent of those miles to bicycles would save at least $100 billion."

Much of urban travel is already "bike-sized": 40 percent of all trips in the United States (and 50 percent in Britain) are 2 miles or shorter. More than 25 percent of all trips are under a mile in the United States. "Cycling could eliminate some of these short, air-polluting trips," Gardner said, citing estimates that 90 percent of emissions in a 7-mile trip are generated in the first mile before the engine warms up.

In several major cities in the Netherlands, Denmark, and Germany, bicycles now account for 20 to 30 percent of all trips. In many Asian cities, the bicycle's share of trips is even higher, accounting for more than half of all trips in some Chinese cities-more than buses, cars, and walking combined.

In stark contrast, bicycles are used for less than 1 percent of all trips in Canada and the United States.

Gardner cites numerous examples of how cities around the world are encouraging the use of bicycles by making inexpensive but effective changes in their transportation systems: special lanes near intersections and advanced green lights for cyclists in Muenster, Germany; "bike and ride" links with public transportation in Japan; micro-credit programs to help low-income citizens buy bicycles in Lima, Peru; and a public-private partnership in Copenhagen, Denmark, that provides bikes for public use.

Mayors all over the world are finding that bicycles can also fight crime and cut administrative costs. The average patrol car costs 12 cents per mile to operate, while bicycles run for less than 1 cent per mile.

The International Police Mountain Bike Association (IPMBA) estimates that more than 2,000 police departments in the United States, Canada, Australia, Iceland, and Russia have bicycle units, with some 10,000 officers on bikes. Even better, the IPMBA reports that half of the police departments it surveyed saw a jump in arrest rates-by an average of 24 percent-when bicycles were introduced into patrol areas.