At a training session at the World Bank in Washington, DC two years ago, Dr. Kavi Bhalla  from the Johns Hopkins Bloomberg School of Public Health asked attendees to look down at the palms of their hands. The meeting included professionals from all over the world who worked with national and local governments on transport policy and projects. When people hesitantly followed his call and placed their hands in front of their eyes, Dr. Bhalla said “your hands are tarnished with blood”.

This shocking start to the lecture was meant to demonstrate that road planners have been making a grave mistake for 100+ years by using road capacity and speed the key objectives of their work. Indeed, this approach has been a monumental failure. Not only has road construction not improved traffic in urban areas, but it has also increased the number of fatalities and serious injuries. Urban expressways and highways have become “parking lots” during peak hours and deadly traps the rest of the day.

More Road Space = More Congestion, More Fatalities

Mobility does not improve with additional car capacity because of basic economics. Since Ibn Taymiyyah in the XIV Century and John Locke in 1691, it has been clear that demand for a good or service increases as price goes down, all things equal. In road traffic, the principle is the same: when travel time falls, car traffic goes up. Any additional capacity that was gained through road expansion is lost to more traffic due to induced demand  (a.k.a. “rebound effect”) after 3-4 years. As Lewis Munford wrote in 1963: “increasing road width to reduce congestion is the same as loosening your belt to fight obesity.”

At the same time, constructing more urban expressways reduces road safety—particularly in the early stages of a city’s development. As a road is widened to accommodate more traffic, average speeds go up—significantly increasing the risk of fatalities and serious injury. Impact at high speed is beyond the limit of what the average person can survive, putting pedestrians in particular at greater risk. The probability of a pedestrian dying in a crash when hit by a car at 50 kilometers per hour (30 miles per hour) is 85 percent.

Shifting the Paradigm

For many years, road safety plans have placed the burden of responsibility on drivers and pedestrians. These traditional plans insist on focusing on educating road users so that they “abide by the traffic rules.” While this can help, it does not solve the road safety problem, as human beings are fallible.

A new approach to road safety, called “Vision Zero” (since 1997 in Sweden) or the “Safe System Approach” (since 1998 in Australia), recognizes that people will make mistakes, and aims to reduce the effects of our mistakes by designing a safer system. This is a considerable change in perspective: the user is no longer held responsible for crashes; instead, responsibility is shared with the designer, builder and manager of the road.

A Slower City Is a Safer City

One result of this change in thinking is that it is no longer acceptable to design urban roads for high speeds. This approach opposes common proposals for building fully segregated urban expressways as a means to solve congestion. Fortunately, cities—not just countries like Sweden and Australia—are moving past the old car-oriented logic:

• Paris Mayor Anne Hidalgo reduced the speed limit in Paris, France to 30 kilometers per hour (18 miles per hour).
• New York City Mayor Bill de Blasio set the speed limit at 43 kilometers per hour (25 miles per hour).
• Last year, Mexico City Mayor Miguel Mancera launched a new traffic code that focuses on reducing speeds and giving priority to pedestrians, cyclists and public transport users.
• Sao Paulo Mayor Fernando Haddad reduced the speed limit to 40 kilometers per hour on all urban roads, with exception of some large arterials and urban highways. The result has been an impressive 33 percent decline in road fatalities.

Of course, Hidalgo, de Blasio, Mancera and Haddad have all encountered challenges. A common response from the public is that speed reductions are absurd. Commentators blame them for making drivers commute more painful, but the reality is that reducing the speed limit only affects average speeds by 3-5 percent. This means that travel times are reduced just 7 seconds per kilometer (12 seconds per mile). Others claim that lower travel speeds result in higher emissions, when there is evidence that indicates otherwise.

Any city serious about road safety should reduce speed limits. But this is about more than simply posting new signs, as compliance is usually low and citywide enforcement is too expensive. The real solution is cities safer by design. This involves changes in road design, increasing the number of pedestrian crossings with traffic lights on arterials, changing intersection design, narrowing traffic lanes, introducing traffic calming devices like road bumps and raised pavements, providing high-quality, connected sidewalks and bike lanes and installing speed cameras for automatic enforcement. It is not just a matter of “education and enforcement.”

If we don’t lower speed limits in our cities, we are committing ourselves to a future full of traffic fatalities and injuries. It’s time to change that.

This article was published in Spanish as an Op Ed in El Tiempo, on March 4, 2016