Two new developments in urban mobility this spring: the much-ballyhooed Citi Bike share program, launching in May (for real this time), and the introduction of Giro’s Air Attack helmet ($200 at Toga Bike Shop, 110 West End Ave., at 64th St.; 212-799-9625). Weighing in at a featherlight ten ounces, the aerodynamic lid works just as well sheathing the noggin of a Tour de France racer as it does a commuter on the Williamsburg Bridge. The innovation is in the polycarbonate shell, which hovers about three millimeters above the scalp and lets in head-cooling winds while eliminating the need for extraneous drag-inducing vents. A model with a detachable magnetic visor is available by special order for an additional $40.