Earlier this spring, two national organizations that study public transportation teamed up to create a giant map showcasing how hundreds of cities' transit networks cater to different neighborhoods.

It also ranked cities on a ten-point scale based mostly on the accessibility of routes, along with how effectively they get people to where most jobs are located. Houston scored a 6.9, compared to Chicago's high-ranking 9.1, Philly's 8.9 and San Antonio's 5.7, for example.

Late last week, one of those organizations, the New York-based TransitCenter, a group devoted to studying transportation systems, took the data from the AllTransit map, as it's called, a step further and focused on the accessibility of high-frequency routes—as well as how that differs based on the race of the rider.

“[High frequency] is one of the most important features of transit, because most transit in the U.S. doesn't run frequently enough to be practical for people,” said Jacob Anbinder, communications assistant with TransitCenter. “It's not predictable; it's not reliable. The rule of thumb is, if you have to read a schedule, then it's not a transit service that will work for you.”

In Houston, according to TransitCenter's analysis, while nearly 80 percent of people have access to transit at least a half mile away from their doorstep, only 18 percent of them have access to what the map's algorithm considers the highly reliable, “high-frequency” service.

By the AllTransit definition of high-frequency—service running at least every 15 minutes on average, 672 times a week—only one bus route, the Westheimer route, meets that standard, along with all three of Houston Metro's light-rail lines.

As TransitCenter noted in its analysis: “Notably, in cities with fewer high-frequency transit lines there tends to be a greater demographic skew among people who live near quality transit.”

And it considered Houston to be one of the two “most notable” examples of that demographic skew.

Even though black and white people in Houston each have equal walking-distance access to public transportation in general—each making up 24 percent of the pie—36 percent of white people can access high-frequency service while only 19 percent of the black population can. Latinos make up 45 percent of people with walking-distance access, yet only 34 percent access high-frequency routes. (See The New York Times's map of the racial makeup of Houston's neighborhoods here to compare with the above map.)

However, Anbinder cautioned that AllTransit uses a much higher standard for “high-frequency” than most other metrics out there. (One of Houston Metro's board members, Christof Spieler, voiced his disagreement with AllTransit's high-frequency metric on Twitter, actually, saying routes should run every 15 minutes, but only for five days a week and 14 hours a day, resulting in fewer weekly trips.) That means that routes catering to more minority communities that may run frequently throughout the day, but drop off the map late at night, for example, may have been excluded from the TransitCenter's data.

Despite the apparent racial disparity, Anbinder said that Houston's transit is far ahead compared to the vast majority of other southern cities. Most don't offer high-frequency service at all, based on AllTransit's high bar. That includes San Antonio, Austin, El Paso, Atlanta, and Jacksonville. Dallas offers it to fewer than 5 percent of people. Anbinder credited Houston Metro's progress to the New Bus Network, launched in August, and the relatively new light-rail system.

Many residents from the southeastern part of town complained to the Houston Press shortly after the new bus routes were launched in August 2015 that they felt negatively affected by the changes. According to an analysis required by the Civil Rights Act, low-income communities lost access to 12 routes and non-low-income communities gained three. Metro officials had said they anticipated push-back, given many Houstonians would need to amend their daily schedules. But since last fall, Metro says it has made service changes to reflect the wishes and complaints of community members.

As to the results of TransitCenter's latest analysis, Metro did not respond to our questions by press time.

*Update, June 16, 3:10 p.m.: Because Metro uses a different metric to define high-frequency—every 15 minutes for at least 15 hours a day—Metro considers 21 additional bus routes to be high-frequency, which run through a diverse range of neighborhoods. Those routes are colored in red on this map.

"This is where I see flaws with the AllTransit analysis: It values every hour of the day and night the same," said Kurt Luhrsen, vice president of service planning. "Yet the demand for service during those times can be very different. So only in the densest cities is demand for all night service come anywhere close to being 24 hours."