NEW YORK — As Mayor Bill de Blasio stepped from his SUV today for his morning workout, he was immediately met by protesters outside the Prospect Park YMCA in Brooklyn.

Less than a block away and less than 24 hours before, a 4-year-old girl and a 1-year-old boy were killed as they crossed Ninth Street in Park Slope with their mothers — in the crosswalk and with the light — by a driver who began rolling through a red light and then accelerated. The 4-year-old’s pregnant mother, Tony-award winning actress Ruthie Ann Miles of Hawaii, was also struck and remained in critical condition.

In the hours after the crash, de Blasio expressed empathy as a pedestrian and a parent. “This is an intersection, again, we know very, very well,” he said Monday. “We have crossed it many times with Dante and Chiara when they were kids, so this is personal.”

Even so, angry residents and pedestrian advocates seized upon the mayor’s morning routine of driving to the Prospect Park Y on 9th Street — a reliable locus for impromptu protest — to demonstrate today for changes to the street to calm traffic. Organizers did not know if the mayor would come. But he did so around 9 a.m. and stopped to speak with the crowd for nearly 10 minutes.

The absence of charges against the driver inflamed resentments among traffic safety advocates who saw a disconnect between the mayor’s goal of reducing traffic deaths and the seeming indifference of police investigators to what, to all appearances, looked like a heinous crime.

The local councilman, Brad Lander, appeared to channel the outrage of many of his neighbors at the driver, Dorothy Bruns, 44, who remained hospitalized today. Lander said he received a call from the Police Department today saying that her license had been suspended.

“She should have had her license and her car taken away from her long ago,” Lander said, pointing as many did today to publicly available records indicating that her Volvo sedan had been involved in numerous traffic infractions. “Who are the people who are too dangerous to be driving, and how do we make sure that they’re not?”

City records indicated that, in the last two years, the Volvo had been caught on camera violating speed regulations around schools on four separate occasions, and another four times was caught running red lights.

While the violations are tied to the car, they do not link to the driver. A law enforcement official, who spoke on the condition of anonymity, said Bruns had a clean driving record on her license.

The problem for prosecutors, should a case be brought in this crash, is that past behavior is not readily admitted into court.

“There would be a problem using that information against the driver,” said Maureen McCormick, a top assistant district attorney in Nassau County and the former head of vehicular crimes at the Brooklyn district attorney’s office. “As a general rule, priors of any sort are not used against a defendant without a specific finding by the court that its probative value outweighs its potential for prejudice.”

The official said that the investigation was still open but that the police believed Bruns had a seizure while driving. Prosecutors are exploring whether she was cleared to drive. Last month, a Brooklyn district attorney brought manslaughter charges against a man with multiple sclerosis whose doctor told him before a fatal crash that he should not be driving. Bruns could not immediately be reached for comment.

At the protest today, de Blasio promised to “redouble” his efforts on his traffic safety plan, known as Vision Zero, and cataloged increases in police enforcement. City Hall officials pointed to the overall decline in fatalities under the mayor’s plan and statistics that showed the intersection where the crash occurred, at Ninth Street and Fifth Avenue in Brooklyn, had a lower incidence of deaths or serious injuries from traffic compared with other locations.

“Every time a crash like this happens, City Hall, DOT and the NYPD determine what more we can do to increase safety through enforcement, engineering and education,” Seth Stein, a mayoral spokesman, said in a statement. “A full investigation is underway to determine the crash’s cause, and the best course of action to make these streets safer.”

Those waving signs asked for changes to the street, including a protected bicycle lane. Doug Gordon, a television producer and advocate for safe streets who organized the demonstration, said the message of greater enforcement from the mayor — accused by some of having a “windshield perspective” on the transit-heavy city he governs — was not enough.

“At the point that you’re stopping a speeding driver, you’re kind of too late,” he said. “We want to prevent people from being killed, not investigate their deaths after the fact.”

After the mayor’s workout, he and his wife, Chirlane McCray, laid flowers on a makeshift memorial for the two dead children, Abigail Blumenstein, 4, and Joshua Lew, 1. Other local political leaders, including Lander, also put down flowers.

The deaths were also widely mourned on social media by many of Miles’ colleagues, including Kristin Chenoweth, Sutton Foster, Audra McDonald and Lin-Manuel Miranda, some highlighting GoFundMe pages for the families of Miles and Lauren Lew, whose son, Joshua, was killed.

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