What is the busiest road in the country?

I originally drafted this in 2009 and was reminded of it by a recent Room for Debate article in the Times, which pointed out that

if the morning subway commute were to be conducted by car, we would need 84 Queens Midtown Tunnels, 76 Brooklyn Bridges or 200 Fifth Avenues.

which is about the same point I am trying to make here …)

What is the busiest road—the busiest single right of way—in the United States? The Jersey Turnpike? The George Washington Bridge? The Bay Bridge? Any number of roads in Los Angeles? Houston? Chicago? The 401 in Ontario?—okay, that's twenty lanes wide and in Canada.

But the answer is, none of the above. And no other multi-lane suburban monstrosity. In fact, quite arguably the busiest roadway in the United States is five lanes wide—of which two are for parking. And it has sidewalks! It's not particularly what goes on on the street, although the road is often congested. But, still, three lanes? Parking? Presumably traffic lights? And it is busier than dozen-lane-wide Interstates?

There's a lot of pedestrian traffic on the street too. Especially since, every eight or ten blocks, thousands of people disappear down stairways and provide most of the traffic on the street. Of course, the street is Lexington Avenue in New York, and most of the traffic comes from ridership aboard the Lexington Avenue Line. WIth 1.3 million trips daily, the line would, on its own, be the largest rapid transit system in the country, other than New York. With more than 50 trains per hour at rush hour—in each direction—the line has a crush-load capacity of close to 100,000 passengers per hour.

(Oh, yeah, there are some cars and buses and bicycles on the surface, but these are margins of error compared to the capacity underground.)

How many cars would it take to move 100,000 people per hour? Well, let's assume 1.5 people per car at rush hour. That's about 67,000 cars. Various studies have pegged the capacity of a highway lane at about 2000 cars per hour, or more than one every two seconds. Any more and the speed—and then the capacity—drops. (I can't find the source, but maximum capacity occurs at around 50 mph, after which, if you add any more vehicles, speed drops precipitously. So if you are in traffic which begins to drop below the speed limit, get ready to slow further.) Highways are relatively inefficient for their space—the five lanes of Lexington avenue, even if they were a highway, could only handle about a tenth of the capacity of the Lexington Line.

So, how many lanes would it take to move 100,000 people per hour? Well, let's make one more assumption. Crush capacity in the peak direction, and full capacity (100 per car) in the other—150,000 people, or 100,000 cars. The math is rather obvious: it would take about 50 lanes to move the number of cars as one subway line—or about the total number of north-south lanes on Central Park Drive, 5th, Madison, Park, Lexington, 3rd, 2nd, 1st and York Avenues, and FDR Drive.

Or to put it another way, every packed-full, ten-car subway train in New York City (or similarly-full trains elsewhere) is equivalent to a full lane of rush hour traffic for an hour.