Last week's answer

Question: When engineers designed the final section of Interstate 70 through Glenwood Canyon, they faced a major spatial dilemma: the canyon floor was barely wide enough for the Colorado River and an existing railroad line, leaving almost no room for a standard four-lane interstate footprint. To avoid destroying the canyon walls or choking the river, what clever geometric design solution did engineers use?

Answer: They stacked the eastbound and westbound lanes into terraced, multi-level elevated viaducts at different elevations along the canyon wall.

To preserve the natural 12.5-mile canyon, Gruen Associates utilized a design separating the highway into two with the westbound lanes above the eastbound lanes. The dual highways then disappera into 4,000-foot-long tunnels. 

Sources: Gruen Associates