Last week’s post on “The Critical Ten,” the difference between driving at 20 and 30 miles per hour, generated an intriguing conversation over at Streetsblog about what, exactly, science can tell us about perception and speed.
I included this chart (given to me by folks at an engineering firm), but streets.mn writer Alex Cecchini tweeted me an updated take on the difference here:
One of the commenters at streetsblog called my conclusions “pseudoscience,” and maybe they’re right!
Here’s part of the conversation:
I’m sorry, but as a researcher in automotive safety for over 20 years, this article is more exaggerated pseudoscience than actual science. First, visual field, isn’t actually a thing, unless you are talking about Visual Field Loss (VFL). I assume what is meant, or at least what is drawn, is a metric known as Useful Field of View (UFOV). In either case, VFL and UFOV are static, i.e., they don’t actually change with speed. They differ from person to person, and VFL happens as part of the natural aging process. Second, UFOV only refers to the field of view where a driver can perceive objects or change in a single glance, without moving their eyes or head. While there is plenty of research discussing how VFL and UFOV might be related to increased crash risk, we don’t actually drive with our heads fixed forward.
So, then I thought, well, maybe the author was plotting some sort of window of glance distributions. Since we do look around while driving, it’s quite possible that glance distributions vary with speed. The faster you are going, the more often you are going to be looking further ahead towards the horizon. And, there is some evidence in this direction, but I found nothing that specifically compares 20 mph and 30 mph, and comes up such a pronounced difference. The best paper I found that did speed comparison, looked at low (up to 25 mph) and high (45+) speed differences, and the high speed was at least 2x what is depicted in that graph, representing more of an oval, than an actual circle. You would still easily see glances to the pedestrians on the sidewalk. (http://onlinepubs.trb.org/onli….
That being said, the author is correct that there is a reaction time penalty. A 10 mph difference equates to an additional 14 feet traveled per second, so if you happen to be looking at the wrong place at the wrong time, there’s a higher penalty in terms of distance traveled before you see the problem. But differences in modern vehicle stopping distances at either of those speeds is pretty negligible. Really, the research is pretty clear on what the key difference is between 30 mph and 20 mph, and that’s the energy of impact. Kinetic energy is 1/2*mv^2. Speed is squared in that equation. It’s simple physics.