Obstructions betwee 300-500 feet AGL are marked with alternating orange and white bands, and /or checkerboard patterns and may require red, or white, steady, or flashing lights. Cable wire crossings are marked with coloured balls, and addition markings or lights may be provided on the support towers. The airspace is becoming increasing crowed with cellular telephone towers, and windmills in many locations, and these hazards often can be found at levels up to 500 feet AGL.