New Yorkers rode their bicycles to work in record numbers during the transit strike last month — and apparently liked it, as a new survey shows a 21% spike in poststrike ridership.An unexpected benefit of the crippling three-day strike was that casual cyclists realized they could pedal to work every day, said Noah Budnick of Transportation Alternatives, a cycling advocacy group."We're seeing a silver lining ... improvements the city made during the strike encouraged people to keep biking afterwards," Budnick said.While improvements such as car-free streets and designated bicycle parking areas didn't last, Transportation Alternatives found rush-hour bike use remained higher last week at three East River crossings, including the Manhattan bridge (up 36% over prestrike levels), the Brooklyn Bridge (17%) and the Williamsburg Bridge (12%).During the December strike, there was a stunning 500% increase in bicycle use that saw bike shops swamped with newbie riders.Mary Strain, 36, sales director at a nonprofit education company in midtown Manhattan, said the strike got her hooked on two-wheel commuting."I would have never considered it unless it was an absolute necessity," said Strain, who lives in Fort Wadsworth, Staten Island, and totes her bike on the ferry."This way my commute will be the same time, and I get an incredible workout," Strain said. "Plus, I got total hero points at the office" for making it to work during the strike, she said.Veteran cyclists noticed an uptick in new riders on the Brooklyn Bridge, including Marc Agger, who cycles every day from Manhattan to his Brooklyn Navy Yard fish business."It's easy to tell who is new to biking because they're bad," Agger said. "People who've done it for a long time know how to communicate with each other."Cyclists willingly brave bad weather, choking car fumes and the perils of the road — which claimed 21 lives last year, eight of whom were Brooklyn residents.But according to Transportation Alternatives, the top issue preventing more people from cycling to work is that many office buildings bar riders from bringing their wheels inside.