UTA's free fare zone map leaves something to be desired.
It shows the TRAX, but not the bus. When I look at the bus map for downtown SLC, there is a lot of service there! UTA might do better just combining the two maps, given their near-identical extents. Here is my MS-PAINT take on such a map: (Gray line with FFZ acronym)
On a proper mapping program, I'd use a specially dashed line, perhaps using arrow symbols. Using a shader or a hash would obscure the information inside the box.
If UTA wanted to move away from a free fare zone, they could designate certain stops or stations, rather than an area.