In July 2023, New Jersey state troopers who patrol the state’s busiest highways and remote rural roads suddenly began writing far fewer traffic enforcement tickets. The next month, citations for speeding, drunken driving, cellphone use and other violations plummeted by 81 percent across the state compared with the year before.

The sweeping slowdown in enforcement continued for more than eight months and coincided with an almost immediate uptick in motor vehicle crashes, records obtained by The New York Times show.

The duration and scope of the slowdown are unrivaled in modern policing, according to academics who study traffic data and law enforcement tactics in the United States. The reduction in traffic enforcement within the State Police is now the subject of a criminal investigation by the New Jersey attorney general, Matthew J. Platkin, according to five people with knowledge of the inquiry who were not authorized to discuss it publicly.

The drastically reduced levels of enforcement began the week after the release of a report that found glaring racial disparities in road safety enforcement. The analysis evaluated more than a decade’s worth of State Police traffic stops and reopened a stubborn wound in a department that spent a decade under the control of a federal consent decree because of similar patterns of bias.

Troopers who believed that they had been closely adhering to strict rules adopted as a result of the consent decree said the report left them concerned that even small missteps might derail their careers.

“All members are reminded of the dangers of conducting motor vehicle stops,” warned an internal memo sent to troopers by their union president, Wayne D. Blanchard.