Today’s the day for outdoor fun, it being about 900 degrees on the pavement.
I have an aerobic exercise for you, something I like to use to help people get a good, on-the-ground grasp of how much space we cover while we’re trying our damndest to avoid whatever just got in our way.
To play, it helps if you establish a stride length for yourself that’s one metre long. Go ahead, throw the tape measure on the ground, and get used to making a one-metre stride.
OK, that was good enough to keep the neighbour entertained. “Martha, that crazy couple next door’s doing something weird again, come look!”
Now, let’s go somewhere we’ll probably need to stop quickly, like one of the high crash rate intersections in town. You know the ones, whichever town you’re in. Kelowna, consider Benvoulin and Springfield. Nasty.
It helps if you choose a spot with lots of safe(ish) space beside the road, like a wide sidewalk or grassy verge. This way, I’m not as likely to get you hurt while we’re trying to figure out how not to get hurt.
Along the way there, let’s think about the whole issue of stopping distances, and blow up a couple of myths.
First, a bit of Driver/Rider Training 101. Stopping distance, says the Very Smart Guy at the front of the room, is a composite of:
- Perception time, and the distance you cover in that time. How long/far it takes you to notice that elephant crossing the road ahead.
- Reaction time, and that distance. How long/far it takes to move your affected members to the brake controls and begin to apply them.
- Braking distance. This is how far you travel with the brakes fully applied, before you actually stop.
You have to see it to stop for it, which can take awhile, and it takes another little while to get on the brakes. Those bits of time are part of the overall stopping process that we need to consider, to get an accurate picture of how much room you need to stop instead of whacking into something.
These little bits of time are always, inevitably, described as being about three quarters of a second each, or a combined Perception Response Time (PRT) of 1.5 seconds on average. So, at say 60 km/h, (17 metres per second), that’s roughly 25.5 metres.
There is very wide agreement about this.
However. It’s nonsense. The research that gave us those numbers, and the work that’s followed and relied on it, has very significant flaws well described by Marc Green PhD,
The fact is, PRT is hugely dependent on the condition of the person and what’s going on around them. A PRT of 1.5 seconds is not “average.” It’s the best possible scenario of an alert, youthful operator anticipating a hazard, in ideal conditions, with zero distractions or complications, and no physical/psychological compromises.
You’re at that intersection, right? Look around you. How many issues are you going to have to see and consider while you’re approaching it, and as you try to move through it?
Vehicles in motion, vehicles stopped, vehicles maybe not stopping, possible left and right turns, cyclists, pedestrians, status of the light, road surfaces, potholes, sun glare, billboards, sandwich boards, dogs, sirens, horns, loud pipes, visual obstructions.
We’re just getting started.
How are you today? At your absolute physical best, alert, frisky, no vision issues, nothing on your mind, well-rested?
Frankly, not likely, but let’s not belabour the point, which is that real-world PRT is just not going to be 1.5 seconds, as we’ve known, but ignored, for a very long time.
“Whenever the driver is confronted with a complex traffic or highway situation and is required to make choices, judgments, and decisions, his response time may increase to 2, 3, or even five seconds.” (American Association of State Highway Officials, 1973, p278).
Because, for many partly-understood psycho-physiological reasons, the complexities of life make it hard to notice, and hard to figure out, which of the world of stuff around us is going to jump out and bite us, we need to plan on needing far more than that 25.5 metres to get on the brakes.
You paced that out yet?
OK, let’s get walking. I like to start at the mid-point of the intersection we’re approaching, and work backward to see where we need to start identifying our hazard.
I’m going to be really generous, and say PRT is going to be two seconds. We’re at 60 km/h on a four-lane roadway, lane widths are 3.7 metres.
- Perception: 17 M. You’re already seven metres back from the crosswalk.
- Reaction: 17 M. Whoa – way back from the intersection now, hard to see everything.
- Braking: a measured average (ideal conditions), 18 M.
Now, we’re so far back from the intersection, 170 feet back, there’s a ton of stuff that’s going to be actually out of sight, and other stuff is distracting us.
But this, where we’re standing right now, is where our stopping distance in the real world, starts. All the way back here.
Look. Way. Ahead.
This article is written by or on behalf of an outsourced columnist and does not necessarily reflect the views of Castanet.