As Hurricane Matthew approached, officials used all the lanes of highways:

Across swaths of Florida, South Carolina, Georgia, and North Carolina, half the highway lanes have reversed. Traffic engineers call this “contraflow,” the volte-face of normal traffic. Now, on both sides of these roads, vehicles only run one way—away from Hurricane Matthew…

To select the exit routes months or even years before hurricane season, transportation planners turn to flood maps and atmospheric modeling. They predict hazards: wind, storm surge, freshwater flooding. They rely on traffic counts and experience to predict if and when residents will decide to finally leave their homes, and how…

The planners build computer simulations of their predictions, and tinker with the variables—down to specific intersections’ traffic signals—to speed up the process. With a few days notice, some regions choose to evacuate in waves, asking those living at low elevations to depart hours or even days before inland residents.

Rural regions often direct their residents toward one major highway, physically blocking off smaller roads. This undoubtedly results in jams, but some officials would rather have their populations—with their attendant gas, medical, and food needs—bunched together than spread throughout the hinterlands. Metropolitan areas are more likely to shut down an entire stretch of interstate, forcing cars onto side roads until they converge on bumper-to-bumper congestion miles from a flood zone.

It makes sense to use all available lanes going away from the hurricane, especially toward the end when few people would want to go the other direction. I would still be intrigued to see how many police such an effort requires and how drivers navigate on and off ramps going the opposite direction than normal. Even with all the lanes open one way, I imagine the traffic is not moving too fast.

If I remember correctly, reversing the lanes of highways was also on the table during the Cold War to quickly evacuate a major city. You can read a current-day guide to preparing for a nuclear blast here – there is no mention of highways. However, it does suggest more scenarios when people might be asked to evacuate:

Evacuations are more common than many people realize. Fires and floods cause evacuations most frequently across the U.S. and almost every year, people along coastlines evacuate as hurricanes approach. In addition, hundreds of times a year, transportation and industrial accidents release harmful substances, forcing many people to leave their homes.

While people may not think about evacuations much, I don’t think the highway lane reversals are common at all.