Megaregions

"As metropolitan regions continued to expand throughout the second half of the 20th century their boundaries began to blur, creating a new scale of geography now known as the megaregion. Interlocking economic systems, shared natural resources and ecosystems, and common transportation systems link these population centers together. As continued population growth and low density settlement patterns place increasing pressure on these systems, there is greater impetus to coordinate policy at this expanded scale."

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http://www.america2050.org/content/megaregions.html