Massachusetts Traffic Stops Analysis, 2014-2022

Massachusetts Traffic Stops Analysis, 2014-2022

Matthew B. Ross, PhD, Associate Professor of Public Policy & Economics

School of Public Policy and Urban Affairs

Northeastern University

Policy Brief 2023-2

November, 2023

The overarching finding from the analysis of Massachusetts traffic stop data between the years of 2014-22 is that there is strong evidence to suggest Massachusetts police are more likely to stop a person of color during periods where they can more easily discern their race and ethnicity. This study was conducted using a solar visibility analysis, comparing the likelihood that a traffic stop is made of a motorist in the daylight versus darkness. To account for the fact that the underlying driving population varies throughout the day, the analysis focused on windows of time between the earliest and latest sunset of the year and immediately before and after daylight savings. The estimates suggest that Black/African American motorists and Hispanic/Latinx motorists were 2% and 6% more likely to be stopped by Masschusetts police during periods of time when their race was more easily discernable. To put these disparities into perspective, the estimates suggest that Massachusetts police may have made as many as 11,564 and 33,543 excess stops from 2014-22 (approximately 1,284 and 3,727 per year) of Black/African American and Hispanic/Latinx motorists respectively.[1] While the magnitude of the estimates varied from year to year, a statistically significant disparity was estimated in six of the nine years analyzed and for the entirety of the period from 2014-22. Focusing on the most recent three years of data from 2020-22, a total of 33 agencies (11 state police troops and 22 municipal agencies) were found to be more likely to stop a person of color during periods when motorist’s race was more easily observed.

These findings are in stark contrast to the “2020 Massachusetts Uniform Citation Data Analysis Report” released in February of 2022 which reported no statewide disparity as well as only three departments with statistically significant disparities.[2] While the period analyzed in that report is considerably shorter than the 2014-22 period examined in this analysis, there are also several underlying methodological issues with the solar visibility analysis contained therein. In particular, the methodology employed by the researchers deviates significantly from what is broadly considered best practices from the literature on empirical testing of traffic stop data from evidence of disparity.

In light of both of these sources of potential measurement error and their impact on the estimates, this analysis should be considered a very conservative estimate of disparate treatment that most likely underestimates the overall magnitude. There are two sources of potential measurement error in the traffic stop data underlying this analysis which are worth noting explicitly. First, Massachusetts ceased uniform collection of verbal and written warnings in 2002. In traffic stop data from Connecticut and Rhode Island collected over a similar period, warnings represented 49.74% and 53.99% of total traffic stops. Since the scientific literature suggests that persons of color are more likely to be involved in pre-textual traffic stops, it is likely that the omission of verbal warnings from this analysis is potentially biasing the estimates away from finding discrimination, even if it exists. Second, analysis of the surnames recorded in the underlying data by reporters at Gannett suggests intentional misreporting of Hispanic/Latinx motorists as White non-Hispanic. Systematic measurement error in terms of race/ethnicity is also likely to be biasing the estimates contained in this analysis away from finding discrimination even if it exists.