ON A RECENT SUNDAY, Brandon Jones, a 44-year-old fund manager at 9W Capital Management, traveled from his home in downtown Manhattan with his wife and two children to meet friends for brunch in Williamsburg, Brooklyn. They were heading to Reynard, the popular restaurant in the neighborhood's fashionable Wythe Hotel, where Manhattan-bound Town Cars regularly idle on the street outside.

But Mr. Jones did not drive. Nor did he take the subway. Instead, he piloted his two children via the deck of his Yuba Mundo, a so-called "longtail"...