Earlier this month, New York’s Taxi and Limousine Commission released data on every cab ride taken in the past year and a half. Photograph by Xavier POPY / REA / Redux

In a squabble with Uber last month, Mayor Bill de Blasio claimed that the ride-sharing company’s growing fleet of cars was choking New York City’s roadways and impeding traffic. He proposed—and then abandoned, after it proved extraordinarily unpopular—a cap on the company’s expansion. Uber countered by blaming the slowdown on lower speed limits, more bike lanes, and more pedestrian plazas. City Hall has agreed to undertake a four-month traffic study, but in the meantime, can New Yorkers measure how much speeds have actually changed?

Earlier this month, something new and exciting happened: the Taxi and Limousine Commission (T.L.C.) released data on every cab ride taken in the past year and a half. Besides being a big step forward for the city’s Open Data initiative, the release may help answer our question, since taxis are good proxies for the rest of the New York's vehicles. Using the distance travelled and the time taken for each trip, plus some elementary physics, we can compute the average speed of traffic.

I took the new data, appended it to five years’ worth of prior data that was obtained as part of a Freedom of Information Law request, and out came a six-year view of the taxi fleet’s performance. Then, for each day, I plotted average speed, accounting only for trips between 0.1 miles and fourteen miles, to avoid really long highway jaunts.

The resulting graph is a little hard to interpret. The sharp peaks and valleys are due to weekends being generally less trafficky than weekdays. (There is an especially large spike during Hurricane Irene, presumably because the roads were empty.) The top three fastest days, on average, were Christmas, the Fourth of July, and New Year’s Day. As many New Yorkers already knew, major holidays are one of the few good times to drive. But what does this chart tell us about the over-all speed of cabs over time? Not much. To fix that, I made the smoother graph below, which also represents average daily speed. The difference here, though, is that we're looking at a rolling average: each data point takes the prior year into account, which has the effect of filtering out the day-to-day extremes.

The plot shows an unexpected trend: speeds were actually improving between 2011 and mid-2013. What caused them to deteriorate? Uber has claimed that the city’s lowering of the speed limit, from thirty miles per hour to twenty-five, is a major cause of increased traffic, so I’ve added a dotted line over November, 2014, the month the change took effect. It’s hard to know how much the new limit played a role in changes to traffic flow, but there certainly was plenty of slowing down going on before it was introduced. The first three months of 2013 saw an average speed of 13.8 miles per hour, whereas the first three of 2014 saw an average of 12.9 miles per hour, a nearly seven per cent decrease in one year. As for Uber’s claim about bike lanes, those have been expanding for a long time. Three hundred and sixty-six miles were added in the seven years before the slowdown began, even as average traffic speed increased.

What about de Blasio’s claim that Uber is to blame? In 2014, the rate of for-hire vehicles being added to our streets almost doubled, to around eleven thousand two hundred and fifty per year, so we would expect to see traffic speed suffering particularly in the past year or so. The graph below plots how quickly that speed was changing each day. (Nerds will know this as the average slope.)

When the line is above zero, taxi speeds over the previous year have risen; when it is below zero, they have declined. At the start of 2013, cabs were getting faster by about 0.0015 miles per hour per day. By mid-2014, they were getting slower by about 0.0013 miles per hour per day—or about one mile per hour every two years. In other words, every day, cabs were getting slower less quickly than they had the previous day, even as Uber was expanding its fleet. This is the opposite of what we would expect if for-hire vehicles were the main force behind falling traffic speeds.

So what can we say? Well, first, raw data is important. Second, traffic is an incredibly complicated thing, and the people who argue about it are prone to overstating their cases.