Here's why there are giant arrows across the US

In the 1920s, the US government started delivering mail by plane. But planes could only travel during the day. Delivering a letter from coast to coast required a plane and then a train that would travel at night.

So the government created the highway of lights, a system of light beacons and large arrows that guided pilots. 1,550 beacons stretched across 18,000 miles. Maintaining this system of giant concrete arrows cost $US23 million a month (adjusting for inflation).

Before the highway of light it took 83 hours to get a letter across the US. After the system was put into place it took just 33 hours.

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