The U.S. Federal Highway Administration has an interesting web site on roadside vegetation management. It lists the Showy Milkweed under its Roadside Use of Native Plants database for Arizona. There is a caveat at the bottom of the web page:
Because the noxious weed lists have continually changed since we gathered them in 1994, we are not including them at this time. Not all States have noxious weed lists. Those that do, do not use the same standards of importance and are not comparable. States typically have included plants that interfere with agriculture (Canada thistle), or cause human health problems (poison ivy). Some States are now including a category of plants that invade and degrade the environment (purple loosestrife). Check with your State's Agriculture Department or Weed Scientist listed below. The noxious weed list can be used two ways on roadsides: 1) check to not inadvertently plant these invasive plants, and 2) note the plants you are legally responsible to control. Many States now check adjacent State lists to avoid planting their neighbors' problem plants. Because weeds do not respect political boundaries, and because by their very nature weeds continue to adapt and expand, monitoring and controlling invasives at State borders is a wise part of vegetation management.