A single visit to Manhattan makes it pretty obvious that most people there depend on the subway, buses, bikes, cabs, and walking for transportation. Just 16 percent of people who commute into Manhattan for work do so by car — by far, the lowest percentage of any US city.

But what would Manhattan look like if everyone drove into the city instead of taking public transport?

According to Vancouver highway engineer Matt Taylor, the island would need 48 new bridges that would each have to carry eight lanes of traffic:

Taylor arrived at that number by noting that 2,060,000 people commute to Manhattan daily. Under ideal conditions, a single lane can convey about 2,000 vehicles per hour, so to let 2.06 million cars on to the island within a four-hour period, you'd need at least 380 additional bridge lanes — or roughly 48 new eight-lane bridges.

Of course, you'd also need somewhere to put all those extra cars. Taylor calculates that they'd require about 24 square miles in total, which is exactly the land area of Manhattan. In other words, you'd need to build a layer of underground parking that takes up the entire borough to fit all the cars driven in by commuters.

Sure, all this is pretty speculative (and it assumes that all commuters would be driving in alone, including the 70,000 students — some of whom aren't old enough to drive — that commute in daily).

But the thought exercise is still a pretty stark reminder of how efficient public transport is at conveying tons of people. Even if you built all those bridges, there wouldn't be enough roads to route the 2.06 million cars underground to the imaginary layer of parking, among many other problems.

In other words, Manhattan as we know it simply wouldn't exist if all its commuters drove.

Further reading: New York City lowered its speed limit to 25. Other cities should do it too.