When open space officials planned to open Mt. Umunhum, a 3,486-foot Bay Area peak that has been closed to the public for 31 years, they expected 20,000 to 40,000 people would drive or bike up each year. But crowds have been visiting “Mt. Um” in droves, at a pace that will bring total visits to 150,000 for the year. The Midpeninsula Open Space summit features expansive views and a Cold War-era radar tower.