September 20, 2007
Reaction Times and Driving - A Back-of-the-Envelope PSA
On Tuesday a man was killed just north of Springfield along I-55 and two others put in the hospital when a southbound car veered across the median and slammed head-on into a semi truck, which then jackknifed across the highway. This is yet another all-too-grim warning that, when driving, you need to keep your head on a swivel at all times and be totally aware of your surroundings, even on a ’safe’ drive.
I’ve driven that part of I-55, both north and southbound, more times than I care to count over the last four months. It’s your classic safe, boring road; no turns, no variations in speed, and most of the time, not a whole lot of traffic compared to, say, the Chicago area. It’s very, very easy to step your alertness down there.
Let’s be conservative and estimate that both vehicles were going 60 miles per hour (from my experience out there, the semi was probably going 60 and the car around 70 when he hit the median, and let’s estimate that crossing the median at that speed loses him 10 mph). That gives us 120 miles per hour relative speed, or, from my loose estimate on the back of an envelope, 10,560 feet per minute, which comes out to about 176 feet per second relative speed.
Put yourself in this situation. You’re John Q. Driver out there on I-55 northbound, after a long day of work, settling in for the long drive up to Chicago, which is about four hours north with traffic at this time of day. Maybe you’re fiddling with the radio, on the cell phone (even handsfree, because you’re a careful driver, of course), but you’re definitely hitting about 70 mph and planning on staying there. In short, you’re not at a heightened mental state.
Driver reaction time, according to this website, is ‘typically’ about 1.5 second from a totally relaxed mental state to beginning physical action to react to an entirely surprising event (with the huge caveat that ‘typical’ time depends on age, time of day, road conditions, and the individual), but let’s assume if anything that it’s a little longer because this is such a ’safe’ road to drive on. At 176 feet per second closing rate, that’s 264 feet before you’ve done a thing, and that’s if you were watching the road to pick up the distraction ahead right as it happened. It takes another .2 seconds or so to move your foot from the accelerator to the brake, and another .3 seconds to push the brake all the way down, so in 2 seconds, or 352 feet at the absolute minimum before you start to take any corrective action whatsoever.
352 feet is a pretty good distance. We’re talking about the length of a football field, from the back of one end zone to another. Now think of all the times you’ve been out driving in a safe area; maybe you turn your head to check traffic behind you, maybe you look down to scan your speedometer or look at the radio. Let’s say it’s another second before you see the situation develop in front of you. Suddenly, going from 2 seconds to 3, you’re looking at a distance of over 500 feet between you before you can see, react, and take action; keep in mind that, in either of these cases, you’re still dead in the end because you didn’t react fast enough to let any reaction take effect and either slow you down or get you out of the way. If you’re over 60, if you’re driving at night, if any other number of things are the case, it could go up to 4 seconds and a whole lot of distance.
You may also be jerking the steering wheel at this point, but let’s call that a 50-50 proposition at best; instinct is to steer away from the impulse, so if the other car is cutting across your path from the left side, your body will want to jerk the wheel right, a move that will probably just put you right in his path and ensure that he broadsides your driver’s side door instead of hitting you head-on in the engine block. If you’re a good driver and have trained yourself to turn into skids, however, hopefully the same instinct would kick in, turn you towards the left, and send you into the median, buying you time to react or at least crash somewhat more safely (hitting something at a lower relative speed than 120 mph, at least).
Cars are heavy, dangerous machines, and drivers typically travel at speeds that would make a head-on collision instantly fatal if they’re driving anything smaller than a semi. Because of the speeds and reaction times involved, the ONLY WAY to keep yourself safe, even on a long, boring road with little traffic, is to keep your head on a swivel, keep yourself alert, and be ready to act to keep a situation from developing instead of trying to react once it has developed.
This message brought to you by the Department of Protecting Yourself from Crazy People Whenever Possible.
Filed under: Column
No Comments


2 Comments