Arrow Boards

Adds arrow boards for road work.
These arrow boards are intended to (more or less) comply with the 2009 MUTCD section 6F.61 specifications for a type A arrow board, and support the following modes: off, flashing arrow (left, right, or dual), sequential arrow (left or right), sequential chevron (left or right), flashing caution (horizontal line or four corners), and alternating diamond caution.