Why It's Hard to Drive at Night After It Rains

Tuesday, January 01, 2013
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Welcome to Giacomo’s Blog @ Kinetic River!

In this blog space I plan to cover a wide range of topics, spanning optics, biomedicine, technology, and other topics I think you might find of interest. The aim is to seek to entertain, enlighten, and stimulate thoughts. Your feedback is welcome, as are suggestions for future topics.

In this introductory piece I want to discuss why it is hard to drive at night after it rains. The answer holds some perhaps surprising insights into the behavior of light. Well, let’s start by comparing some things. For example, driving in the rain is generally harder than driving in fair weather, for fairly obvious reasons: raindrops on the windshield, poorer tire traction, etc. But I am not interested in these at the moment (even though they do make it harder to drive). I’m interested in what happens even after it stops raining: for a while at least, the road is wet. Beside the already mentioned skidding hazard, what makes that condition difficult?

Fig. 1:  The difference in nighttime driving between dry and wet
road conditions.

Well, during the day, not much, by itself. In fact, a daytime drive after the rain clears the air of smog and other particulates can be absolutely delightful—desirable, even. But at night it’s another story. “Well,” you might say, “it’s obvious: it’s dark! Of course it’s harder.” Yes, but that doesn’t explain why it should be different depending on whether it has just rained or not.

The answer lies in optics. When you drive down a dry road at night (for simplicity, let’s start with the case of just you on it), unless you have a death wish, you’ll have your headlights on. That’s because you want to see where you’re going! What happens, of course, is that the light from your car beams floods a swath of road in front of you, and the road surface sends some of that light back to your eyes. Indeed, it doesn’t preferentially choose your eyes: it sends some of that light in all directions, and the direction that just happens to meet your eyes is responsible for your seeing where you’re going. This very common phenomenon, so common most people don’t think twice (if that) about it, is called diffuse reflection. And it’s a good thing (in this context). It is caused by the roughness of the road surface: irregular contours, dirt, small pebbles embedded in the asphalt, etc., all contribute to this randomized “reflection.”

What happens after it rains? Well, the rainwater tends to form flat pools in the nooks and crannies of the pavement; even where it does not pool, it coats the irregular surfaces of the road, resulting in a wet surface that’s a lot smoother than what’s underneath. When light hits a smooth surface (as opposed to a rough one), it bounces off neatly and predictably: the subject of countless high school physics experiments the world over. This is called specular reflection (from speculum, Latin for mirror). And this, too, is a good thing, although unfortunately not in the context of driving at night on a wet road! Because specular reflection means that the light from your headlights is, mainly, going to bounce right off and move on forward, away from you. It won’t come back to your eyes, except in small measure (from those coated pavement surfaces which happen to be oriented just right to direct the reflected light back to your eyes; or from devices meant to do that, like cat-eyes and roadside reflectors). As a result, the road is a LOT harder to see, making driving quite a bit more difficult.

Fig. 2: Reflections from dry and wet surfaces.
Pooling water tends to turn rough surfaces into smooth ones,
and diffuse reflection into specular reflection.

Driving at night on a wet road is made even more difficult by an additional consequence of specular reflection. Just like the light from your headlights bounces off the road and moves forward, so does the glare from other cars’ headlights—except that, in their case, “forward” means “in your eyes.” With dry conditions, the only significant sources of light you have to shield your dark-sensitized eyes to are the oncoming cars' headlights. With wet conditions, you effectively have a doubling of the problem.

Some solutions exist to mitigate these issues. For example, 3M now offers for sale all-weather pavement marking products. These do increase road visibility; however, they don't affect the extra oncoming-car glare. A somewhat rough pavement surface would be naturally impervious to specular reflection, and it wouldn't require additional installations; careful design could also reduce or eliminate unintended potential consequences like higher noise generation from the tires of passing cars.



Image credits: Flickmor (dry road); US DoT (wet road); The Physics Classroom (diagram)

Comments

Karen Dippel commented on 15-Jan-2013 02:24 PM
Never understood this phenomenon before. Explained very well, thank you.
Bill Gilman commented on 22-Feb-2013 09:31 AM
Welcome to the blogsphere! Another topic along these lines is to explain driving during snowfall at night. I know you don't see too much snow out in San Jose, but here in Rochester, NY we get to see a lot of it. Often at night, and especially during heavy snowfall, you'll notice that you can see better and farther with your low beams as opposed to your high beams. I suspect this is due to the low beams illuminating fewer of those pesky snowflakes who reflect the light back to you. High beams result in many more bright snow-spots of light in your field of vision. More interesting is that at times you can see better with no headlights on at all! You see relatively few snowflakes, and the snowcover on the ground produces enough diffuse reflection from natural light sources that a dark-adpated eye can see quite well. Of course, cars coming the other way can't see you, but that's another story! Cheers, Bill
Giacomo Vacca commented on 25-Feb-2013 11:48 PM
Bill, thanks for the comment. You're right, not too much (any) snow here in San Jose. But as I grew up in Northern Italy, I am totally familiar with your observation about high beams in heavy snowfall. Incidentally, a similar phenomenon occurs with heavy fog, which in the Po Valley is very common in winter and fall. So perhaps fog lights might work better than low beams during snowfall, too? I don't recall trying. As for going "dark," I think I'd have an issue with that if I was driving an oncoming car... Thanks for reading!
Elwood Jones commented on 22-Nov-2015 09:03 PM
I have driven sans headlights on a rather deserted country road during a snowfall event and it is amazing how well you can see. It was so eerily quiet and I felt like I was the only person on Earth. What an incredible feeling!
shafi rr commented on 01-Dec-2015 10:37 PM
Good explanation.... i am a teacher, i will be using this one while explaining regular and diffused reflection. thnx a lot.
Giacomo Vacca commented on 01-Mar-2016 03:42 PM
@Elwood, thank you for your comments. I know exactly what you mean about the difference driving in the snow without headlights. Has its own perils, and not recommended, but clearly the headlights do not provide the best visibility when there is a lot of backscatter.
Giacomo Vacca commented on 01-Mar-2016 03:43 PM
@shafi, thank you very much. Glad you could use this!
Felicia commented on 20-Oct-2016 11:17 AM
In reference to specular reflection - are there glasses that can mitigate its effect?
Giacomo Vacca commented on 13-Nov-2016 07:56 PM
@Felicia, there are indeed glasses that mitigate the problem of specular reflection--otherwise known as "glare." Polarized sunglasses do that. They work by eliminating one of the two states of polarization of light, and specular reflection (from horizontal surfaces) happens to be mostly horizontally polarized, for typical driving conditions. So polarizers oriented to pass only vertically polarized light will cut the glare down significantly.

The problem is that polarizing sunglasses typically are... sunglasses! They're meant to be used during the day, when light is plentiful, so they not only eliminate one polarization, but also reduce the amount of light in the remaining polarization. They really can't be used at night for that reason.

It would be an interesting idea to use true polarizing glasses (meaning ones that only eliminate one polarization, but leave the other one unaffected) at night. I haven't tried. Caution must be urged, though, because the light that you do want to see (the diffuse light from your headlights, coming back from the road and other objects) will be unpolarized--and will therefore be cut in half by the polarizers. Since light at night is already low, cutting it in half may not be wise.
john commented on 12-Jan-2017 09:56 AM
After driving in the rain last night, I noticed an increase in visibility, when I stopped driving on a blacktop (asphalt) road, and started driving on a concrete (cement) road.
Giacomo Vacca commented on 16-Jan-2017 11:01 AM
@john, thanks for the comment. Concrete (especially on roads, where it gets constantly bleached by the sun) is generally gray to very light gray, in contrast to asphalt which is very dark gray to black. So, all things being equal, you'll get more light reflected back from concrete than from asphalt just because of their colors. That holds true even when the surface is wet: While the overall amount of light that's reflected is much diminished in both cases, concrete will still reflect more than asphalt.
Mikey commented on 04-Nov-2017 08:29 AM
Can you come to Singapore and explain to our traffic police and Land Transport Authorities why rougher asphalt pavement could be better than using super smooth black tar to improves visibility at night.
Giacomo Vacca commented on 04-Nov-2017 03:34 PM
@Mikey, I'd be happy to!

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