From today's issue of Parade 'magazine' (in the Sunday paper):

Q: What has gone wrong with air travel?

A: (from Marion Blakey) We're in the safest period in aviation
history, but some of our current system is built on World War II
technology. **Planes must fly on specific highways in the sky to
maintain contact with the ground.** It costs time and fuel and is
inefficient.
[emphasis added]