What do hold bars outline at an airport?

Hold bars at an airport are specifically designed to outline the boundaries for runway edges and centerlines. They serve as visual aids for pilots to identify where they must stop their aircraft in order to maintain proper separation from active runway areas. These markings are crucial for maintaining safety during taxiing, as they ensure that aircraft do not inadvertently enter the runway without proper clearance from air traffic control.

The importance of hold bars cannot be overstated, as they help prevent incursions that could lead to potential collisions with other aircraft or vehicles operating on the runway. The other choices, while related to airport operations, do not accurately define the purpose of hold bars, which are focused on runway access and safety protocols.