Is Twenty the New Twenty-Five?

Q: I’ve noticed some cities changing their speed limits in neighborhoods and business districts from 25 MPH to 20 MPH. Does five miles per hour really matter for safety or is it just another way to discourage driving?

A: There is little doubt that some city planners would like less people driving cars on city streets. And not just city planners; when I drive through Seattle I often selfishly wish that less drivers were allowed on the road (as long as I’m still allowed, of course.) New roads are expensive and there isn’t always room for them; in crowded cities fewer drivers might be the best (even if unrealistic) option. However, I don’t think a reduction from 25 MPH to 20 MPH is part of that agenda. The data show that a five MPH speed reduction really does have an effect on pedestrian safety.

Technically, politicians set speed limits, but when done right, they make those decisions based on the recommendations of traffic engineers. The traffic engineers are tasked with the paradox of moving people as efficiently as possible and getting them there safely. Their job would be much easier (but worse for drivers) if they could just pick one or the other. Your city prefers efficiency? No speed limits and no stop lights for arterial streets. (With all the tow trucks and aid cars responding to crashes on those streets, it might not really be that efficient.) Rather focus on safety? Make all the speed limits 5 MPH. In reality, traffic engineers live in between the extremes, striving to meet both goals simultaneously.

There’s more to safety than speed limits, but that’s what this question is about, that’s what we’ll focus on today. There are two parts to vehicle speed and how it contributes to crashes; the likelihood of a crash and the severity of a crash. Let’s start with likelihood.

Our ability to avoid a crash is dependent on, among other things, having enough time to react to an incident. Imagine that a pedestrian steps into the street 75 feet away from you. At 20 MPH on dry pavement it’ll take about 63 feet to stop your car. That includes the roughly 1.5 seconds it takes to recognize the hazard and for your brain to tell your foot to step on the brake. At 25 MPH, you’ll travel another 22 feet, for a total of 85 feet, before stopping. Eleven of those 22 feet are traveled during that 1.5 seconds while your brain is deciding what to do. At 20 MPH it’s a close call, at 25 MPH it’s a collision. When the city of London reduced speed limits from 25 MPH to 20 MPH they had a 46% reduction in collisions.

You may argue that I chose a distance of 75 feet so I could make my point. And you’d be absolutely right. If I’d chosen 50 feet instead, the pedestrian would have been struck in both examples. But that brings me to the second part of the speed issue; crash severity.

When hit by a car, pedestrian survival is directly related to the speed of impact. A traffic study in the UK (the Europeans are way ahead of us in traffic safety) found that humans have a 95% survival rate when struck at 20 MPH, but that drops to a 55% rate at 30 MPH. At 40 MPH, only about 15% of pedestrians survive a confrontation with a car.

It’s time that we admit what the psychologists and engineers already know; that our efforts to eliminate driving errors through education and enforcement have not been entirely successful. Those are two core components in traffic safety, but we also have to consider how we design our transportation system. Psychologists who study human error will tell you that no amount of training or experience will completely eliminate errors, in driving or any other parts of our lives. Transportation planners now give more thought to building systems that are designed to reduce the consequences of those inevitable errors. With that in mind,  some cities have adjusted their balance between efficiency and safety by changing speed limits from 25 MPH to 20 MPH.