Chicago Transit Authority ridership has plunged since mid-March, when Illinois’ stay-at-home order went into effect. But the declines in train and bus ridership were much smaller on the city’s South and West sides compared with the North and Northwest sides.

A WBEZ analysis of CTA data from March shows that South and West side residents continued to use mass transit at much higher rates than people in other parts of the city.

The CTA said Sunday that ridership systemwide fell 75% to 80% in March compared with normal levels, and has remained down more than 80% in April. The WBEZ analysis of March data shows that ridership plummeted 84% on trains and 71% on buses in the week after the stay-at-home order went into effect on March 21, compared to the same week last year.

The March data show ridership began dropping in the days just before Gov. JB Pritzker’s stay-at-home order, then took a dive after that.

Although the decline was across the CTA’s system, the data reveal disparities in how ridership fell in different parts of the city.

A same-day comparison of normally busy Red Line stations on the South and North sides provides a snapshot of the differences. On March 30, the last Monday of the month, the Red Line’s 79th Street station on the South Side saw more than 2,300 entries. That compares with the line’s Fullerton station in Lincoln Park, which had only 765 entries that same day. Last year, Fullerton averaged 11,800 rides on Mondays in March.

Entries were also far lower March 30 at stations in Wicker Park, Logan Square and Lakeview, compared with normal ridership.

Also, the CTA data reveal that South Side neighborhoods with higher ridership are the same areas with significantly higher rates of COVID-19 deaths. WBEZ reported earlier this month that 70% of Chicago’s COVID-19 fatalities were among African Americans, although they are only 29% of the city’s population.

The CTA did not give an explanation for the plunge in ridership the past two months. But the trend has clearly occurred during a pandemic when many commuters began working from home, and other people have lost their jobs or were furloughed.

Still, the March data show that many people in some parts of Chicago continued to depend on CTA service.

“Although overall CTA ridership is down, CTA is still providing close to a quarter-million rides per day,” the agency said in a statement on Sunday. “Public transit is an essential service, and it’s important to keep trains and buses running for all Chicagoans who rely on the service.”

Even before the stay-at-home order went into effect, Mayor Lori Lightfoot began her now-familiar mantra of practicing social-distancing. But images of crowded buses on the South Side began to circulate on social media, raising questions about the feasibility of keeping six feet apart on packed buses.

Afterward, the CTA began to allow drivers to limit the number of passengers they picked up and to use rear-entry boarding on buses.

Bus ridership on South Side routes still remains higher than on routes running downtown or on the North Side, the CTA data show.

The viral images of overcrowded buses on the South Side were on the 79th Street route, which had almost 10,000 riders on March 30, much lower than usual. But the normally busy Belmont 77 route on the North Side had less than half that many riders — 4,000 — on the same day.

“While this is an extremely difficult time for all of us,” Lightfoot said in early April. “I am heartened by the resilience of our bus and train operators who our communities rely on every day for transportation, especially our South and West Side neighborhoods that have been hit the hardest by this pandemic.”

Elliott Ramos is WBEZ’s data editor. Follow him @ChicagoEl.