Last week, New York City officially lowered its default speed limit, from the standard 30 mph to 25. That difference may seem arbitrary and hardly worth noting, but it actually makes a big difference when it comes to saving lives. For one, cars going more a bit slowly will have an easier time avoiding crashes in the first place. But the real difference is a huge jump in pedestrian survival rates when crashes do happen: The laws of physics and human anatomy translate to 30 mph being far deadlier than 25 mph. The difference in velocity translates to the car lifting pedestrians off the ground, and creating traumatic impacts against vital areas like the head.

“I’d estimate that a person is about 74 percent more likely to be killed if they’re struck by vehicles traveling at 30 mph than at 25 mph,” says Brian Tefft, a researcher with the AAA Foundation for Traffic Safety who wrote a 2011 report on the subject. He looked at 549 vehicle-pedestrian accidents occurring across the US between 1994 and 1998, accounting for factors like vehicle size and pedestrian BMI. The risk of serious injury (defined as likely to result in long-term disability) for a pedestrian hit at 23 mph was about 25 percent. At 39 mph, it jumped to 75 percent. Analyzing his findings, Tefft says, “25 to 35 mph, they’re almost three times as likely to be killed.” 35 mph, he found, was the median impact speed for fatal pedestrian crashes.

A 2010 study in London had similar findings: “In all of the pedestrian datasets, the risk of fatality increases slowly until impact speeds of around 30 mph. Above this speed, risk increases rapidly – the increase is between 3.5 and 5.5 times from 30 mph to 40 mph," the author, D.C. Richards, writes.

So why doesn't a 20 percent change in speed just mean a 20 percent change in serious injuries? There are lots of variables at work here (is the car an Escalade or a Fiat? is it a direct hit or a side swipe?), but, it turns out, the 30 mph mark is something of a limit for what our bodies can live through. Above that speed, organs and the skull aren't necessarily strong enough to withstand the kinetic impact of a bumper and windshield.

"It has to do with fracture forces," says Dr. Peter Orner, a licensed physician and former engineering professor who consults on injury biomechanics in car crashes. "As velocity increases, you're crossing thresholds." Though he's skeptical of the comprehensiveness of studies like Tefft's, Orner also says that at higher speeds, "the car is going to scoop them up." And when you're talking about cars, what gets scooped up is usually smacked against a windshield or thrown onto the ground. That can easily lead to brain trauma.

The good news is that cars have gotten safer over the past few years, and federal regulations designed to protect pedestrians in the event of a crash are making a difference. But then throw in rise of distracted driving and yes, distracted walking, which has led to a recent spike in pedestrian injuries, including those not involving automobiles. So it seems like a good idea for NYC to make everyone drive a bit more slowly.

Whether the speed limit change will make for safer roads is up for debate. “If the actual [car] speeds are reduced in response to the change in the speed limit,” Tefft says, “it should have a safety benefit.” But that’s not always the case. “In general,” he says, “the research shows that it takes more than the number on the sign to change the speeds of traffic.”