San Diego’s first wheelchair-accessible hiking trail opened Wednesday near Rancho Peñasquitos, giving the elderly and people with mobility challenges a chance to see views and natural habitat previously unavailable to them. The trail, located in the Black Mountain Ranch Open Space Park, is relatively short at just over one-fifth of a mile. It’s flatter and wider than most trails and it’s made of decomposed granite because people using wheelchairs and other mobility devices typically struggle on dirt trails.