A blog about fog
Do we drive faster when our vision is impaired? As eLife, a new open-access online journal, has officially launched this week, here’s our take on some research they published in advance that shows what happens to our visual perception when faced with fog.
Fog is one of the more difficult conditions to drive in. Even Father Christmas would struggle if he didn’t have his handy red-nosed reindeer to guide his sleigh. But what about those of us who just need to get home? Researchers used to believe that people drove faster in low-visibility conditions because they underestimated their speed. But a group at the Max Planck Institute for Biological Cybergenetics in Germany has now found that the visual system of our brain actually tells us to do the opposite in the fog.
Previous studies used a simulation of fog where visibility was reduced evenly across someone’s area of vision. This is a good way of simulating a windshield that has steamed up or is dirty, but it’s not good at simulating real fog. Fog is distance dependent: it makes visibility increasingly poor the further away an object is. This distance-dependent visibility was what the team in Germany wanted to test.
The group used state-of-the-art virtual reality to see how fast drivers would travel in various visibility conditions. The drivers were first shown a scene to simulate them travelling at a certain speed. They were then asked to say what speed they thought they were travelling at, at different levels of visibility.
The researchers could show that, in distance independent visibility, like a steamed-up windshield, drivers underestimated their speed and therefore went faster, as expected. But with distance-dependent visibility, like real fog, drivers tended to overestimate their speed and slow down. This went against what was previously thought.
To really test their theory, the group wanted to see what results they would get if the distance-dependent visibility was reversed, a condition they called ‘anti-fog’. In this simulation, visibility became clearer the further away an object was: the opposite of normal fog. The team found that drivers underestimated their speed and got faster in response to ‘anti-fog’. This showed that not only did it matter whether visibility was distance dependent, but also in what direction the dependence was in.
The researchers have shown that changes in visibility cause us to perceive speeds differently, and therefore to change the speed we drive at. But often in real life, people speed more in bad weather conditions. If this isn’t because our visual system is telling us to, then perhaps people speed simply to get home quicker. Although this may seem like a good idea at the time, it’s obviously not the safest way to get around. So next time you’re driving on a really foggy night, remember to listen to your visual system and slow down.
Pretto P, Bresciani JP, Rainer G, & Bülthoff HH (2012). Foggy perception slows us down. eLife, 1 PMID: 23110253