Cycle commuting is hot. Warm, at least—depending on where you’re living.

Each year, the League of American Bicyclists, a nationwide cycling advocacy organization, takes a look at the annual commuting numbers out of the American Community Survey. The ACS is a product from the US Census Bureau, and if you’re into bicycles, it asks one really good question every year: “How did this person usually get to work last week?” The League of American Bicyclists took last year’s responses—as they have for the past five years—and broke them out by city to answer another good question: Where is American cycling growing?

There are some caveats: The ACS data doesn’t capture the number of folks who are cycling for fun or to run errands. (Note: the number of bike-share trips were up dramatically last year.) People who ride to a bus or train station might only report the public transit leg of their commute. The data also might not take into account those who cycle to work one or two times a week, instead of every day. And because it limits respondents’ answers to a single week, it might not capture people who pedal seasonally, strategically avoiding a bicycle commute at the sweaty height of summer or frozen depths of winter. (The Census Bureau solicits survey responses from about 3.5 million Americans throughout the year.)

All that said, in 2017, according the ACS, the share of commuters cycling to work actually dipped by 4.7 percent compared with the previous year. Less than 1 percent of American commuters regularly use their bicycles to get to work. But 84 percent of the seventy largest cities in the US have seen an upward cycle commute trend over the past 12 years.

The most interesting aspect to these numbers—and certainly not a new one—is the uncovering of a profound cycle commuting gap. In the five US cities with the highest share of cycle commuters (Davis, Santa Cruz, and Palo Alto, all in California, along with Boulder, Colorado, and Somerville, Massachusetts), an average 11.7 percent took bicycles to work last year. But in the next five (Cambridge, Massachusetts, Berkeley, California, Miami Beach, Florida, Portland, Oregon, and Ames, Iowa), just 7 percent do. Take cities 20 to 25 (Redwood City, California, San Francisco, Bloomington, Indiana, Portland, Maine, and Salt Lake City), and just 3.1 percent of those cities take bikes to work. You’re either a cycling city, one that opens its streets to two-wheeling—or hardly one at all.

“I shouldn’t be surprised, but I’m always a little bit surprised by the difference between the regions and just how far ahead Western cities tend to be compared with every other region,” says Ken McLeod, the League of American Bicyclists’ policy director, who wrote the report. In the West’s top 20 cycling cities, an average 5.9 percent of commuters cycle to work. But just 2.2 percent of workers pedal to the office in the Midwest’s top 20 cities; it’s 2.1 percent in the South. Maybe most surprising of all: in America's East, known for its dense, urban places that should be hospitable to cycling, just 2.5 percent of those in the region’s top 20 cycling cities actually ride to work.

The chasm seems to be a function of city investment. “In most, if not all places that have sustained increases in bicycling commuting, there have been investments in bicycle infrastructure—roadways that account for people on bikes and people walking,” McLeod adds. “Those places have tried to reduce speeds and make driving safer, too, so people feel safer while biking.”