We examine the effect on the incidence of casualties and crashes of a city-wide vehicle speed limit reduction in New York City (NYC) streets. The law change, part of Mayor Bill de Blasio’s Vision Zero Action Plan to improve traffic safety, cuts the default speed limit for streets with no speed limit signs from 30 to 25 mph beginning November 7, 2014. We use a monthly panel dataset with crash statistics for the entire population of NYC streets, from July 2012 through March 2019. Several difference-in-differences regressions show a statistically significant and meaningful decline in injuries and crashes.
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Tingvall and Haworth articulate that crashes with lesser (not long-term disabling) injuries or no injuries are beyond the scope of Vision Zero—reducing those types of crashes is not a goal of the approach. However, in practice, it is plausible that changes in speed made to reduce the severity of injuries from crashes may also reduce the total number of crashes and the number of lesser- or no-injury crashes.
As described in Belin (2016), “‘System designer’ is a diffuse concept but it refers to any of the public and private agencies responsible for the design and operation of various parts of a transportation system, including roads, vehicles, and public transit services, and those responsible for any support systems, such as laws and regulations, education and public awareness, surveillance, rescue, and care and rehabilitation. State and municipal road-maintenance authorities, vehicle manufacturers, driver education programs and schools, private transportation companies, and healthcare providers are among the other stakeholders included in the definition of system designer.”
The previous NYC mayor, Michael Bloomberg had implemented programs and street design changes to increase safety and improve conditions for pedestrians and bicyclists (NYC-DOT 2013).
The Vision Zero Network is a national nonprofit advocating for Vision Zero programs. Its criteria for designating cities as Vision Zero are: “(1) sets clear goal of eliminating traffic fatalities and severe injuries, (2) Mayor has publicly, officially committed to Vision Zero, (3) Vision Zero plan or strategy is in place, or Mayor has committed to doing so in clear time frame, and (4) key city departments (including Police, Transportation, and Public Health) are engaged.” (Vision Zero Network 2019).
The program broadly referred to as Toward Zero Deaths (TZD) has been promulgated by 30 US states since 2001. Munnich et al. (2012) find that the four longest-standing programs, in Idaho, Minnesota, Utah and Washington, have been effective in reducing traffic fatalities. They note, “Successful TZD programs have five characteristics: (1) an ambitious goal of eliminating traffic fatalities and serious injuries; (2) high levels of inter-agency cooperation in pursuit of the TZD goal among state departments of transportation, public safety, health, and other relevant agencies; (3) a comprehensive strategy addressing all 4 E’s—engineering, enforcement, education, and EMS (emergency medical services) elements of traffic safety; (4) a performance-based, data-driven system of targeting resources and strategies where they will have the greatest impact in reducing traffic fatalities; and (5) policy leadership from relevant entities, including the Governor, the state legislature, and the heads of state agencies.”
As one would expect, in low- and middle-income countries, weak legislation and enforcement, and lack of resources are often major impediments to improvements in traffic safety (Peden et al. 2004).
A speed limit reduction from 37 to 31 mph in urban areas of New South Wales differed from this study, and the other Australian studies in that participation by local governments was voluntary and reduced speed limits were denoted with extensive signage; crashes and speeds were reduced (New South Wales Road Traffic Authority (NSW RTA) 2000, as cited in Hoareau et al. 2006).
Three additional academic studies are similar to the current investigation in examining the effect on casualty crashes of speed limit changes in urban areas. In Oslo, Norway, Elvik (2013) found a temporary reduction from 50 to 37.5 mph on arterial roads reduced casualty crashes by 25–35%. In Hong Kong, Wong et al. (2005) found speed limit increases of 31–43 mph and 44–50 mph increased casualty crashes by 1–36%. In London, England, Grundy et al. (2009) found a 42% reduction in casualty crashes where 20 mph zones were implemented. These studies differ from ours in being implemented at selected sites with signage changes. In addition, the first two studies considered urban highways (NYC’s highways were not affected by the NYC speed limit reduction), and the London study considered “zones” where speed limit reduction is accompanied by substantial traffic-calming engineering interventions.
The NYPD Motor Vehicles Collision Data compiles information from Police Accident Reports (form MV104-AN) filled out by an officer at the crash scene. Officers are required to report on all crashes where fatalities or injuries of any level of severity occur. If a crash victim has suffered multiple injuries, only the most severe injury is listed in the report. Officers assess injuries either by observation (for example, if the victim is unconscious) or from information reported by the crash victim (for example, if there is no visible injury but the victim reports pain or nausea). (NYS-DMV no date, pages 1 and 26). Therefore, in this paper “casualty” refers to a fatality or any injury observed by, or reported by the victim to, the officer at the crash scene.
Speed limits remained constant throughout the study period on the control streets. The control streets include larger streets, such as limited access highways or major arterial streets, with posted speed limits of 30 mph and above; and some smaller streets, such as those near schools, which are signed for speeds less than 25 mph. (NYC Vision Zero 25-MPH-faq 2014). The majority of NYC streets are unsigned (see Table 2); this is the treatment group. The statutory speed limit on these streets was 30 mph prior to November 7, 2014, and 25 mph thereafter.
Polling results in the 1-year report indicate that in October 2014 prior to the law change, 30% of New York drivers accurately identified the default 30 mph speed limit. The city conducted “25 MPH Outreach” in the fall of 2014, with street-team in-person distribution and mailings of publicity materials, social media postings and radio ads. In December 2014, 62% of New York drivers correctly identified the new 25 mph speed limit (NYC-MOO 2015, p. 46). Campaigns are ongoing regarding the 25 mph speed limit and its safety benefits (NYC-MOO 2019, p. 18, 68).
We perform the same exercises for Fixed-Effect Negative Binomial estimation, and a pooled Zero-Inflated Poisson estimation (since one cannot use fixed effects in ZIP without manually enforcing them), see Weber (2014) for more details. The results remain similar in significance and direction.
This 38.7% is calculated by: (− 0.174/0.45)*100, where 0.45 is the average number of casualties on treated streets (Table 2, row 1, column 1).
This 35.8% is calculated by: (− 0.648/1.81)*100, where 1.8 is the average number of crashes (Table 2, row 4. column 1).
This 27% is calculated by: (− 0.174/− 0.648)*100.
We omit the decomposition into different types of victims here for brevity, since we show the aggregated totals, but these estimates with leads are available upon request.
In addition to implicitly capturing the announcement effect in the leads, we tried to isolate it exclusively. Creating a separate difference-in-differences for the announcement on October 2014 in the eight estimations from Tables 3 and 4 does not meaningfully change the coefficients on our primary D-I-D, the speed limit change. Furthermore, in the eight specifications tested, the D-I-D for the announcement effect was insignificant in all specifications except for the count of crashes, where it was significantly negative. These results are available upon request.
NYC-DOT (2019) discusses city-wide congestion and low travel speeds, particularly in Manhattan.
Earlier studies of highway traffic sometimes found little effect of speed limit reductions on mean travel speeds (e.g., Parker 1997).
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The authors would like to thank Jason Barr, João Pereira dos Santos, and two anonymous referees for very helpful comments and suggestions. Carly Dennis and Emma Joslyn provided excellent research assistance.
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Mammen, K., Shim, H.S. & Weber, B.S. Vision Zero: Speed Limit Reduction and Traffic Injury Prevention in New York City. Eastern Econ J 46, 282–300 (2020). https://doi.org/10.1057/s41302-019-00160-5
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