Neal Jennings / Flickr

They fell out of favor decades ago due to safety concerns, but that may finally be changing.  

Every weekday morning in San Francisco, four lanes of traffic whiz across the Golden Gate Bridge and into the city, while just two lanes move in the opposite direction. After the commute calms down, however, one of those lanes undergoes a transformation. With help from a new and clunky yellow machine, the concrete median slides over, leaving three lanes into the city and three lanes out. Normally, traffic adapts to road conditions, but in this case, road conditions adapt to the daily rhythm of traffic.

Zipper Truck in Action - Courtesy: Golden Gate Bridge District.MP4 from Golden Gate Bridge District on Vimeo.

For anyone who commutes during rush hour, it's easy to forget just how much road space goes to waste. Both streets and bridges are built to fit far more cars than are usually on the road. They're designed to accommodate the daily spike of drivers during peak hours. “Our roads are overbuilt for 18 hours a day, and on the weekend,” says Laurence Lambert, a traffic engineering consultant at Stantec.

In other words, if we didn't all drive at the same time, we wouldn't need all 4 million miles of road that currently exist in the U.S. But since we're unlikely to scrap the 9-5 work day anytime soon, traffic engineers face a challenge: How do you fit more cars into a fixed amount of road?

This is where bridges like the Golden Gate provide a helpful illustration. It's extremely difficult to widen a bridge, which means that during spikes in traffic, engineers need to increase capacity without increasing the total number of lanes.

Almost a century ago, reversible lanes were proposed as a solution to this problem. They took advantage of the fact that—thanks to the development of suburbs—city traffic moves a lot like the tides. Every morning, cars flood into America's cities. Every evening, they flow back out. As a result, at rush hour, 75 percent of traffic gets squeezed into one half of the road. A movable center divider can fix that problem by freeing up unused lanes.

Reversible lanes were an elegant solution, but they proved unexpectedly tricky to implement. The Golden Gate Bridge, for example, has six narrow lanes that are crossed by 3 million vehicles every month, so every inch of road counts. Reversible lanes were installed there more than half a century ago—but that meant that twice a day for all those years, road workers had to manually move the lane markers to show drivers the direction of each lane.

The Golden Gate's original reversible lanes, which debuted in 1963, made traffic jams rarer and less severe. The problem was that they also proved deadly. Because the median needed to be flexible, it was marked off with yellow plastic cylinders instead of a sturdier metal or concrete barrier. There was no protection if a car veered across the median—and in fact, there were so many head-on collisions that the middle lanes were dubbed “suicide lanes.”

The old, more problematic method of reversing a lane on the Golden Gate Bridge.

It took decades for the bridge authority to finally approve and fund the installation, last January, of a movable barrier machine known as a “zipper.” Twice a day, it travels the 1.7-mile length of the bridge, pushing a heavy concrete and metal barrier across one lane. “It's very unusual,” says Paul Grant, a spokesman for Barrier Systems, which built the machine. “There's something going on that you've never seen before.” The total cost to the bridge authority—including equipment, training, and labor—was $30 million.

Both these facts—the danger of the initial method, and the difficulty of installing the new one—help explain why the rest of our roads aren't more efficient. When America’s suburbs expanded during the 1960s, reversible lanes became a popular addition to highways and arterial roads (which are wide enough for lots of traffic, but still allow turns into driveways, parking lots, and intersections). But their track record was spotty.

“The devil is in the details with these things,” says Brian Wolshon, an engineer at Louisiana State University who co-authored a paper on traffic lanes with Lambert. Because drivers make split-second judgments based on instinct and memory, reversible lanes created new logistical challenges. With roadways changing daily, even small operations—like a left turn into a driveway—could become hazardous. Meanwhile, signals and barriers needed to be re-designed, to ensure that they informed drivers of changes.

New approaches to transportation very often face resistance. Wolshon points out that if a new technology makes roads more dangerous, engineers face public backlash—but if they reduce every person's commute by 10 percent (i.e., 3 or 4 minutes) most people won't even notice. He says this makes traffic engineers reluctant to try new things. “In general we're risk averse, and very unwilling to innovate.”

By the 1970s, it was clear that some reversible lanes were creating more problems than they were solving. They were installed effectively on certain highways, where engineers had control over the entry and exit points of vehicles. Some cities, like Washington D.C., also managed to install reliable signals and barriers that kept drivers safe. On the whole, however, “they kind of fell out of favor,” says Laurence Lambert.

Today, that may finally be changing. Reversible lanes have proven their worth in certain situations, for example during large sporting events and evacuations. They're cost effective where construction is expensive, like on bridges and in dense cities. Meanwhile, new technologies are making reversible lanes easier to implement. Remote-controlled LED lighting might improve safety by communicating more clearly with drivers. And movable barrier systems—like the one guiding traffic on the Golden Gate—are improving, though such equipment doesn't come cheap.

There are other ways to fit more cars into the existing road system, of course. On some highways, the shoulder opens to certain vehicles during rush hour. More broadly, some city planners now advocate for less segregation between residential suburbs and business-focused cities. If they get their way, commutes could become shorter and less directional, putting less pressure on existing roads.

Either way, we shouldn't lose hope in a more efficient network of roadways. Reversible lanes are a reminder that innovations require time, money, and risk. Though we typically think of infrastructure as static, even a highway can be dynamic—it can adapt to the needs of the human beings who use it.

Lambert believes that, with budgets shrinking and construction costs increasing, reversible lanes are coming back into style. Utah is one of several states to recently install “flex lanes” on its highways, on the grounds that they increase road capacity without requiring new construction. “The days of pouring concrete are limited,” says Lambert. “You'll have to do more with less.”

Perhaps there's still time to turn our traffic problems around.

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