* Number of cycle commuters has doubled in 10 years
* Berliners weary of transport troubles and fuel costs
* Prominent Germans among those bitten by the bicycle bug
By Erik Kirschbaum
BERLIN, July 31 (Reuters) - Every morning, lawyer Peter Kupisz hops on his bicycle and rides to his office, joining a fast-growing crowd of 500,000 who pedal along Berlin's cycle paths and boulevards each day.
Deteriorating rail services, high petrol prices, climate change worries and a desire for exercise have combined to make commuting by bicycle in Berlin more popular than ever, with an estimated 13 percent of all trips done by bike.
Defying Germany's reputation as a car-mad country, Kupisz said he looked forward to his daily dose of slow-speed exercise. It takes him 15 minutes to ride the five km (just over three miles) to and from his law office. He sometimes even cycles to court.
"It's mainly for the exercise but it's also just as fast as the underground," said Kupisz, 37, perspiring slightly but smiling broadly on his way to work one recent morning.
"I started biking about two years ago but took a few months off at one point and really found myself missing it. So I started again and ride pretty much every day now. It's a part of the day I really look forward to."
Even if the German capital lags behind cycling havens such as Amsterdam and Copenhagen, the number of bicycle commuters has doubled in the past 10 years and should double again in the next decade, according to Sarah Stark, head of Berlin's ADFC cycling association.
Breakdowns in the city's S-Bahn rail service -- hundreds of carriages have been taken off the tracks for safety checks taking months -- and an underground strike earlier in the year have driven thousands more commuters on to their bikes.
"The rail problems and the strikes get people to try commuting by bike and a lot of them discover 'hey, this works pretty well'," Stark said. "There's been a real increase in the last year or two. Biking to work has definitely become trendy."
A rise in fuel prices in 2008 had a similar effect on bike commuting, with many sticking to two wheels even after prices fell again, the ADFC said.
"Sometimes there are so many bikes backed up in the bike lane at traffic crossings that it takes two green light phases to get across," said Stark, who commutes 11 km each way to work.
Across Germany, there are an estimated 4 million bike commuters. About 9 percent of all journeys are made by bike, according to the Transport Ministry. That is still far below 27 percent in the Netherlands and 18 percent in Denmark.
The ADFC believes the number of bike commuters in Germany could rise to 11 million in the next decade.
"The numbers are growing by the month," said Stark. "What we've noticed is that a lot of people switch to bike commuting each time fuel prices rise or the trains get hit by strikes."
The growing number of cyclists on bike lanes next to the main boulevards attract new converts, she added.
"There's a giant explosion of interest," Stark said. "People sitting in a traffic jam see the bikes filing past and it makes them think twice about sitting in their cars."
There are 68 million bikes in Germany, a country with a population of 82 million. The total has risen by 2 million in three years. Bike sales have also remained strong in the past year, defying the economic crisis.
Cycling is a big industry with annual turnover of 1.7 billion euros ($2.40 billion) for cycles and a total of 3.5 billion euros for equipment. It employs about 9,000 people.
In Berlin's government quarter, hundreds of journalists, lobbyists, politicians and businessmen can be seen on bicycles at any time of the day between April and October. There are a growing number of die-hards who ride through mild winters.
Some prominent Germans take advantage of Berlin's bicycle-friendly wide avenues and bike lanes to pedal to work.
Deputy Finance Minister Joerg Asmussen and Martin Wansleben, managing director of the Chambers of Industry and Commerce, are regular bike commuters.
"We're always looking for ways to improve the infrastructure for cyclists to make it even a more attractive alternative," Berlin's Economy Senator Harald Wolf said in an interview.
The city spends 3 million euros ($4.24 million) a year improving 600 km of cycle paths and lanes.
"There are several big advantages of getting around by bike in a city like Berlin," Wolf told Reuters. "First of all, it's a cheap way to travel, and secondly it's positive for the environment because there's no CO2 emissions.
"On top of that it's healthy and in cities like Berlin that have so much traffic the average speed falls so far that you're almost as fast on a bike as you would be in a car."
Rolf Dieter Peschel is another happy cycle commuter. He sold his car and bought a bike two years ago. The 48-year-old, who now rides about 6,000 km per year, reckons it has saved him about 3,000 euros each year.
"I feel a lot fitter than before," said Peschel, who works as an ambulance medical assistant and commutes 17 km each way to work. "I look forward to my bike ride home all day. It's really amazing how much money you can save without a car."
Bettina Krause, 40, paused near the end of her 30-minute, 10-km journey to work at the Technical University at a bike lane traffic light, where 50 cyclists waited for a green light.
"I'd rather take my bike than squeeze into a crowded train," she said. "The thing I like about biking to work is that you can integrate a workout right into your day. I can't stand getting into the train any more."
Thomas Geithner, a 32-year-old IT worker, said he left his car at home most of the time because the bike was faster.
"It's only about four km each way so I'd end up spending more time looking for a place to park than it takes to bike," he said. "There's just too much stress with the car. I'm here on the bike in a few minutes and in a good mood all day." (Additional reporting by Caroline Copley; editing by Andrew Dobbie)