The Mount Sanitas Trailhead serves the trails of Mount Sanitas and the trails on the north side of Red Rocks/Settler's Park. The Mount Sanitas Trail is extremely popular, and this trailhead is often full. People come here at all hours of the day (and night) to walk their dog, hike, run, boulder, and socialize.
From Canyon and Broadway, in Boulder:
Drive, ride, walk, or roll north on Broadway 5 blocks to Mapleton. Turn left (west) and climb steeply up Mapleton. Follow Mapleton for 6 blocks (about 0.6 miles) to the intersection of Mapleton and 4th. Start looking for parking here in case the lot is full. Continue past the hospital about 0.3 miles and you'll find a small trailhead on the right. There is only parking for about 8 cars here. There is also a bike rack to lock your bike.
There is overflow parking a couple hundred yards up Sunshine Canyon, on the left, and all along Mapleton, east of the trailhead.
The RTD bus #201 will get you within a couple blocks of the trailhead, near 4th and Mapleton.