In the vast majority of incidents, Mr. Lander told me, drivers do not get their licenses revoked until they are charged. To get Ms. Bruns’s license taken away temporarily required petitioning a judge, even though city records indicated that during the past two years, the car she was driving had been caught on camera violating speed regulations around schools four times and running red lights on several other occasions.

It is possible, of course, that Ms. Bruns was not driving the car in the moments of those infractions, but it is also possible that she should not have been driving at all. The police believe she may have had a seizure while driving; she was taken to the hospital after the crash where she remained.

The tragedy points to the limits of Vision Zero, a plan to reduce pedestrian fatalities and one of the mayor’s signature initiatives. It has been largely successful: Pedestrian deaths dropped by 32 percent last year, making 2017 one of the safest on record. Speed cameras that have been shown to reduce crashes involving fatal and serious injuries are in place in some parts of the city. But automation in this sense has a real cost; it removes the presence of a police officer who could pull a speeding driver over and see if something might be off. Beyond that, Vision Zero cannot get around the broader spirit of the law, which treats cars much in the way it treats guns, sanctioning the right to operate them as nearly sacrosanct.

For five years now, this truth has been evident to Thomas Wallack, an investment manager who lives in Brooklyn Heights. On a February afternoon in 2013, Mr. Wallack was on the phone with his wife, Martha Atwater, as she was leaving a bakery in the neighborhood when the line went silent. A few seconds later he received a called from a UPS driver who had witnessed her get hit by a car. Mr. Wallack went to the hospital where she had been taken to find that she was dead. He went home to tell his children and then walked over to the scene of the crash to see what he could learn.

“I said, ‘Where’s the guy?’” Mr. Wallack recounted recently. He was told that the driver had been released, that the state trooper in charge had done an on-site evaluation and determined that the driver could go. No drug or alcohol test was immediately performed, Mr. Wallack said. “He was never charged; he got his car back; he didn’t even get a ticket.’’ He had, the police said, a blackout related to diabetes.