These arrows were once used as navigation for the U.S Air Mail pilots. These were active in the 19th century. The arrows would also have a light beacon beside them and a small metal hut for the people who worked at the location. The light beacons would have a small light in them to help guide the pilots during the night.
The arrows are now considered as landmarks and were stopped being used when radar systems started to appear.
More information and history in the links below.