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Reduced Speed Limits Save Lives In Busy Cities, Study Shows

A new study from the University of Illinois showed that in São Paulo, Brazil, speed reductions significantly reduced fatal accidents while increasing travel times only a little bit.

A new study from the University of Illinois showed that in São Paulo, Brazil, speed reductions significantly reduced fatal accidents while increasing travel times only a little bit.

The study researched the São Paulo speed limit program and noted that the program reduced accidents by 21.7% — 1,889 accidents were seemingly averted in the span of 18 months.

The study used a dynamic event study design along with measurements of 125,000 traffic accidents, 28 million traffic tickets issued by monitoring cameras, and 1.4 million repeat observations of real-time trip durations before and after a regulatory change.

“We estimated that the social benefits of accident reductions greatly outweighed the social costs of increased travel time. Not only that, but it’s also a pro-poor policy that mainly benefits low-income residents,” said one of the authors, Peter Christensen, an environmental economist at the University of Illinois Department of Agricultural and Consumer Economics (ACE).

The Study

In Brazil in 2015, the city of São Paulo reduced speed limits on highways from 55 miles an hour to 43 miles an hour (90 km to 70 km per hour). It didn’t stay this way for long, though — in 2017, the new mayor reversed the policy, a promise made on the campaign trail. This gave researchers a unique opportunity to compare accidents at three points: before, during, and after the drop in the speed limit.

As noted at the top, they found that over a period of 18 months, accidents decreased by almost 22% on roads that were impacted by the policy, equating to not only 1,889 accidents being prevented, but also 104 fewer fatalities than would have been expected otherwise.

The study also discovered that 86% of the benefits from reduced accident damages involved low-income residents — mostly pedestrians and motorcyclists. As someone who has never driven or owned a car before, I am not surprised. It’s rough out there for pedestrians.

“Our results indicate that speed limit reduction can be a very efficient life-saving policy. In São Paulo, gun violence is the other major cause of unnatural death. It’s about the same magnitude as road accidents. Our study shows speed-reduction policies have a much bigger effect in terms of reducing fatalities than any policy we’re aware of to reduce violent crime,” Christensen explained.

“And motorcycle transport is a widely used transport option for low-income households in many developing countries, giving us reason to believe that these policies will disproportionately benefit poor households in urban areas around the world.”

Christensen also pointed out that speed-reduction impacts require data-intensive work, which is why such research is rarely done in cities located in developing countries. This proves problematic, since the World Health Organization (WHO) notes that actions in developing countries are the keys to reducing fatal accidents worldwide.

How The Research Was Conducted

Christensen’s research group specializes in analyzing large data sets, and this allowed them to conduct this study, which is the first on this particular topic. They gathered information about accidents form the São Paulo Traffic Agency that included these details about each accident:

  • Victim demographics
  • Severity of injury
  • Type of vehicle

Christensen explained that the group identified the exact location of every accident and the information on travel times. They also researched congestion in the areas that were impacted by the policy. “The data that are becoming available for analyzing these policies are completely transforming the precision of analysis and recommendations coming from the research community,” he said.

The group’s first study was whether the lower speed limit would reduce accidents. “It does — by a lot,” Christensen noted. The second question was whether or not the reduction in accidents outweighed the social cost of increased travel time in the highly congested city. It took the team 9 months to develop the software that allowed them to analyze trip durations using data from Google Directions API.

“With this methodology, we can measure precisely how conditions change on arterials affected by the policy because it’s coming from real-time information; from cars moving on the road,” Christensen explained. After analyzing 1.4 million trip observations in 2017, before and after the speed limit reversed, the researchers found that travel times were 5.5% higher when the reduced speed limit was in effect.

Then they estimated the impact of trip delays by multiplying the increased time with the value of each person’s time. For example, if a normal trip takes 20 minutes to get from point A to point B, and then an additional 4 minutes is added, then that’s 4 extra minutes of someone’s time. “Economists estimate that value for all of the people who are impacted. So even relatively small impacts per capita can add up since over 20 million people are living in South Paulo and using transport,” Christensen said.

Christensen concluded that “The benefits certainly exceeded the costs in São Paulo. Given the magnitudes, we think it’s likely that they’ll exceed the costs elsewhere, though of course, this depends on a number of factors. Other governments should do their due diligence with planning before implementation, but our findings indicate that this is a tool that can be used to dramatically and cost-effectively mitigate the primary cause of unnatural deaths in the world today.”

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Written By

Johnna owns less than one share of $TSLA currently and supports Tesla's mission. She also gardens, collects interesting minerals and can be found on TikTok


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