Here's the relevant narrative from the EA on this specific alternative (with one strike-through edit on my part to fix a typo they didn't catch):
Control Timing of Use
Under this alternative, the NPS would spread use out over the day by assigning hikers to specific time slots. Controlling the timing of use would eliminate midday crowding and maintain freeflowing conditions. A ranger would be stationed at the base of the cables and would serve as a gatekeeper allowing hikers to use the cable system only during their assigned time slot. Chapter 2: Alternatives January 2012 Half Dome Trail Stewardship Plan EA 2-15
This alternative was dismissed because it does not meet the purpose of increasing safety along the trail corridor. A late ascent time could have pose safety risks for many hikers. Most people plan their hike to Half Dome to take advantage of all daylight hours. Forcing some users to wait to ascend until the late afternoon would result in an increase in the number of hikers. Potential consequences of having to wait for a later ascent time could make their day even longer. There have been numerous search and rescue incidents caused by completing the return portion of their trip after dark. In addition, delaying people until later in the day means they are at the summit in the afternoon when thunderstorms are more likely.
CaT
