Being struck by a car or truck is the leading cause of injury-related death for children in New York City. Within the last few months, our City has mourned along with the families of nine-year-old Cooper Stock, eight-year-old Noshat Nahian, and three-year-old Allison Liao, among others. Pledging to prevent these tragedies, in just his second week in office, Mayor de Blasio stood with grieving parents and announced his vision for eliminating traffic deaths in New York City. "There is an epidemic of traffic fatalities, and it can't go on," he said. "And the time to start change is now."

Of all the measures being considered to achieve the Mayor's vision, none holds more promise than the push to reduce speed limits on City streets.

Lower speeds save lives. When drivers aren't speeding, they have more time to brake when something unexpected happens. Lower speeds mean that even when a driver or pedestrian makes a mistake, there is often time to avoid a crash and, if there is not enough time, the penalty is in most cases not death.

The science is clear: If a pedestrian is hit by a speeding driver traveling at New York City's default speed limit of 30 mph, there is a 30% chance that person will die. That number goes up to 80% if the driver is going 40 mph, as too many motorists do. But at 20 mph—the speed limit we should have—there's a 98 percent chance that same pedestrian will live.

Researchers have also uncovered a startling developmental fact: children under 15 are biologically incapable of accurately perceiving the speed of an oncoming car if it's traveling faster than 20 mph. This fact is why we hear so often about children "darting" into traffic. Studies show that every 1 mph reduction of vehicle speeds on urban, pedestrian-heavy streets leads to a 6% decrease in traffic fatalities. And New York City is home to the most pedestrian-dense streets in the country.

It's not just academic research that has shown that it's time to slow traffic down. Cities as diverse as, Paris, Tokyo, and Columbia, Missouri have as well. In the City of London, the rate of crash-related deaths and serious injuries within 20 mph residential zones dropped by 46%. If New York had Tokyo's traffic fatality rate, we'd have 100 fewer deaths per year, and the number of people seriously injured by drivers who don't have time to stop would plummet as well.

But New Yorkers don't need statistics to convince them that the city's streets are too hazardous for their families because of rampant speeding. After a recent spate of crashes that have killed several children, people in neighborhoods across the Five Boroughs are demanding this commonsense safety measure.

Some claim that slowing down traffic would actually cause more crashes, because some drivers would not obey the new 20 mph limit, leading to dangerous "speed differentials." While this has occasionally been known to happen in highway settings, prompting authorities to set minimum speed limits, it does not apply to the urban streets that make up most our city.

In urban areas, the speed differentials are between vehicles going at or above the speed limit, and those traveling at slow speeds because they are doing things like preparing to stop, or getting ready to make a turn into a side street or driveway. If the fastest vehicles slowed down, that would decrease the speed differential, which would save lives.

We can't hold back on lowering speed limits because we fear drivers won't comply. Instead, we need to make sure the placement of 20 mph signs is preceded by public information campaigns, and followed by consistent traffic enforcement, as well as design changes that encourage slower speeds.

The effort must include the overhaul of New York City's most dangerous corridors, which are currently designed in ways that encourage speeding and reckless passing. Measures like "road diets" and devices like speed humps and traffic calming circles have proven very effective in lowering the speeds of the fastest drivers, who create the speed differential that endangers others. Fortunately, Mayor de Blasio's plan to reduce traffic fatalities and serious injuries includes all of those elements of education, enforcement and design.

Georges Jacquemart, P.E., AICP, is a Principal at BFJ Planning and a professor of transportation planning at Pratt Institute