Paul Steely White, the longtime executive director of Transportation Alternatives, will leave the street-safety group in November and join the scooter company, Bird, as director of safety policy and advocacy.

White, 48, directed the city’s crucial bike and pedestrian advocacy organization for close to 15 years. He told Streetsblog he was leaving for “a mix of personal, organizational and professional [reasons].”

As executive director, White oversaw many of TransAlt’s biggest campaigns, including advocating for protected bike lanes, the successful launch of Citi Bike, banning cars from Central and Prospect parks, and championing Vision Zero improvement such as lower speed limits and right-of-way protections. White declined to name his proudest achievement, telling Streetsblog simply, “I made solid progress towards the unfinished task of putting the car in its rightful place: several rungs below walking and other human scale transportation.”

Once seen as a possible candidate for local office, White was clearly more comfortable persuading officials through advocacy and changing the very language of debate. His group, for example, gave the Department of Transportation key political cover to install the Prospect Park West bike lane — a street design that some Park Slopers found controversial a decade ago, but is routine now. In  2011, he won the Rockefeller Foundation’s Jane Jacobs Medal, an annual honor for “new ideas and activism.”

He was particularly eloquent when cyclists or pedestrians were killed by drivers, most recently eulogizing Australian tourist Madison Lyden, who died on Central Park West because “the city failed her.”

Perhaps his greatest legacy is expanding the organization itself, which had a budget of $1 million when White took over, but now has 30 staffers and a $4-million budget. Another legacy: Anti-bike politician Anthony Weiner once called him a “policy jihadist,” a badge of honor.

A Brooklyn father of three, White said he was “looking for new challenges,” but ones that would be “in the same vein of reclaiming streets from cars.”

“I noticed that Bird was making some smart moves, and tapping into enormous latent demand for easy mobility,” he said of the scooter company that is working with the city to have its electric devices legalized and regulated.

“It wasn’t so long ago that streets in this town were little more than car sewers with narrow sidewalks,” he added in a statement. “I’m proud to say that we helped change not only how streets are designed, but we also elevated the expectation of what city streets can and should be. And even though I’m leaving TransAlt, I’m going to continue working every day toward that same goal: to make streets better for people.”

White will be succeeded by a tandem leadership of Marco Conner, who is currently the group’s legislative and legal director; and Ellen McDermott, who is currently the chief operating officer.

They will become “co-interim directors” after White leaves in November, presumably after the annual Vision Zero Cities conference on Nov. 7 and 8.

“The work TA does is so local and it’s our local advocates who people know and love,” White said when asked if the organization would remain in good hands. “There are lots of fresh jedis who can now spread their wings wider.”

White, an often-seen presence on the city’s bikeways, is not the only high-profile defection from TransAlt for the private sector mobility share world. Earlier this year, TA Deputy Director Caroline Samponaro left and soon joined Lyft as it moved towards taking over Citi Bike.