Driving In Fog

Mysterious misperceptions can lead us astray when driving in fog. We explore the science of perceived speed and offer tips for foggy driving conditions.

Atmospheric Obfuscation

More than snowstorms or accidents, the number one cause of highway pileups is foggy conditions. We all understand that reduced visibility makes driving more difficult and dangerous, but fog can be particularly troublesome due to how it affects our visual perception.

First, we’ll cover what you should and shouldn’t do when driving in fog. And then we’ll dive into the complex and sometimes counterintuitive research results surround our perceptions of speed in foggy conditions.

The Dos and Don’ts of Driving in Fog

First, slow down when driving in fog. Don’t use cruise control, don’t pass, and be ready to stop suddenly should an obstacle appear. As we’ll see below, maintaining a consistent speed in fog can be challenging, so it’s important to be aware of how fast you’re going by periodically monitoring your speedometer.

Use your fog lights or low beams, especially at night, for better visibility. Conversely, don’t use your high beams. If you’ve ever driven in fog at night, you know that your high beams put out so much light, which then bounces off the water droplets of fog, that your visibility is reduced even further. Instead, take the rare opportunity to make use of the fancy low-glare fog lights you optioned with your vehicle.

Use your wipers and defrost to ensure good outward visibility. The moisture both inside and outside your car can cling to the windshield, making it even harder to see where you’re going.

Keep your distance from other drivers. This is especially important on busy roads and interstates where following distances are often less than ideal. Foggy conditions often have drivers bunching up as they unconsciously feel safer when they can see the car in front of them. Fight this urge.

On a related note, don’t just follow the lights of the car ahead of you. They might have driven off the road or stopped. Instead, follow the white line rather than the center line to keep you further from oncoming traffic.

Generally, be extra attentive when driving in fog. Turn off the radio and open a window to better hear for traffic. Be prepared to stop at any time. Fog can thicken without warning, so don’t take your current level of visibility for granted.

The Complexities of Perceived Speed

That first bit of advice, to drive slowly in fog, has some interesting nuances worth exploring. Yes, we’re going to get a bit nerdy here, but not super technical.

Research into speed perception when driving in foggy conditions once indicated that reduced contrast/visibility caused drivers to underestimate their speed. The common phenomenon of finding yourself driving faster in fog when you should be slowing down seemed to be explained. However, subsequent research in the last decade has thrown several complications into this picture.

Early research relied on tests of visual speed perception (egospeed) that uniformly reduced contrast, that is, the entire visual field was made opaquer, like a fogged-up windshield. This caused test subjects to underestimate speed and then go faster to compensate. The problem is fog doesn’t uniformly reduce visibility like a foggy windshield. With fog, things in the distance are more obscured than things closer to you, and this has profound effects on how we judge speed.

A more recent study by Paolo Pretto and colleagues compared perceived speed in three scenarios. Foggy conditions, “global contrast reduction” i.e. the foggy windshield used in prior experiments, and a new anti-fog condition, where near objects were obscured while those in the distance were clear. They found that, counter to prior research, foggy conditions actually caused subjects to overestimate their speed and feel they needed to slow down. Meanwhile, both the anti-fog and global contrast scenarios caused subjects to underestimate speed.

Additional research suggests that the relative density of fog plays a role in the perception of speed, but the effects aren’t purely linear. One wrinkle is something scientists call global optical flow rate, that is, how we judge movement based on objects passing by. Obscuring or eliminating roadside objects, like in heavy fog, makes it harder to judge speed. (Interestingly, the more familiar an object, the easier it is for you to judge your speed because you know its actual scale.)

Significantly, Lidestam et al, noted in a 2019 Transportation Research paper that our peripheral vision of close objects is key to our perception of speed. Meanwhile, our primary focal point, typically at distance when driving, isn’t as acute in this regard. In fact, a car itself reduces our perception of speed by obscuring the ground around us. Just compare how fast 25mph feels on a bike or motorcycle versus in a car and you’ll recognize this effect.

So, why do some drivers speed up in foggy conditions? It’s possible they’re compensating for their brains telling them to slow down. A case where the conscious mind is saying, “I’m already trying to drive slowly, so I can’t be going as fast as my brain is telling me, therefore, I should speed up a bit.” It turns out, with fog reducing our field of vision to the peripheral we’re more accurately gauging how darn fast we’ve been driving in our cars all along.

As one researcher observed, our visual perceptions of speed in fog are complex and include variables like the density of fog, visibility distance, global object flow rate, and global contrast (the relative brightness of objects depending on time of day and other conditions). The upshot? Listen to your brain and slow down in foggy conditions.