[Originally posted in May, 2007]
“I just didn’t see him” is a claim that’s repeated over and over in accident reports. Drivers earnestly claim that they simply didn’t notice the bicycle/pedestrian/motorcycle they crashed into. The claim is made so frequently that certainly there must be a grain of truth to it. Yet it certainly isn’t the case that car drivers can’t see such obstacles — after all, they can see traffic signals that are much smaller than a bike or a motorcycle.
What they mean to say is that their attention was otherwise engaged — perhaps by a phone conversation, perhaps by other traffic, or perhaps because they were trying to find something — a street sign, a restaurant, a gas station. Human attention is a fickle thing, and in many cases we don’t notice very obvious details changing right before our eyes.
Consider the following movie (QuickTime required): One image will be displayed for a number of seconds, followed by a white screen, and then a second picture — the same image with one very obvious detail changed. Can you spot the change (don’t cheat–just watch it once!)?
Let’s make this a poll:
I’ll give you another chance at the test later on, so don’t repeat it just yet — the point is that it’s often quite difficult for us to notice changes when we don’t know what to look for.
But what about when those changes can literally mean the difference between life and death? Perhaps when we’re driving a car, we’re so attuned to the possibility of an accident that momentary distractions like looking for a street sign don’t detract from our ability to notice hazards. Steven Most and Robert Astur developed a simple, elegant experiment to test that notion.
They asked volunteers to participate in a driving simulation task. The simulator had a steering wheel, gas pedal, and brakes, and volunteers were asked to navigate through city streets, turning whenever an arrow flashed in a particular color — either blue or yellow. The car drove automatically at a constant speed of 30 miles per hour, and the participants were told not to stop unless they were about to hit a pedestrian or another vehicle.
Drivers had to pay close attention to the arrows, because they could change color as they flashed, and they were only supposed to turn when the arrow stopped flashing and was the correct color — immediately before they reached an intersection. After seven successful turns, a pedestrian walked across the street, forcing a stop. The key manipulation, however, occurred after 9 turns. At this point a motorcycle unexpectedly veered into the driver’s path. For half the drivers, the motorcycle was the same color as the arrows they were supposed to follow, and for half the drivers, the motorcycle was the color of the arrows they were supposed to ignore. Was there a difference in stopping time when the motorcycles were different colors? Here are the results:
When the color of the motorcycle matched the color of the arrows the driver was following, the braking time was significantly faster, and almost no drivers collided with the motorcycle, compared to a whopping 36 percent collision rate when the color of the motorcycle didn’t match the color of the arrows.
Most and Astur conclude that even in a dangerous situation, the attentional set of a driver has an important impact on driving ability. Just as drug users are quicker to spot changes in drug-related items in a photo, so drivers are faster to react to road hazards if they match other items they’re looking for as they drive. So in this case, there’s no difference between a dangerous situation and an ordinary situation; what matters is the particular attentional set at of the driver the time of the incident.
One potential application, the authors suggest, might be on road construction sites: if the color of traffic markers such as cones and reflectors matched the color of worker uniforms and construction equipment, then drivers would be quicker to react to those hazards.
Now back to the demo I started with — can you notice the change in the picture this time? I’m betting a large number of readers will do better the second time they take the test. Go ahead, try it:
Did you see the change this time? Record your response in this poll.
If all goes well, we should see a significant improvement this time. I’ll explain why in the comments — after plenty of readers have had a chance to respond.
[Update: I changed the task to respond to several readers’ suggestions that what we may have is a practice effect. See comments 12 and 30 for details]
Steven Most, Robert Astur (2007). Feature-based attentional set as a cause of traffic accidents Visual Cognition, 15 (2), 125-132 DOI: 10.1080/13506280600959316