# Roads Are Most Slippery Just After it Starts Raining **URL:** \ **Category:** Factual Questions\ **Created:** [July 21, 2025, 8:40pm UTC](https://boards.straightdope.com/t/roads-are-most-slippery-just-after-it-starts-raining/1021159 "2025-07-21T20:40:09Z")\ **Posts on this page:** 19\ **Page:** 1
**Author:** ![jjakucyk](https://avatars.discourse-cdn.com/v4/letter/j/3d9bf3/32.png) [@jjakucyk](https://boards.straightdope.com/u/jjakucyk)\ **Post date:** [July 21, 2025, 8:40pm UTC](https://boards.straightdope.com/t/roads-are-most-slippery-just-after-it-starts-raining/1021159/1 "2025-07-21T20:40:09Z")
This is a frequently repeated claim, that roads/pavement are the most slippery in the first 10 minutes or so after rain starts, because the rain brings dirt, brake dust, tire rubber, and oils to the surface. After it’s rained for a bit, those substances are washed away and traction improves. I don’t doubt that this is true, but it’s often presented as a universal situation irrespective of local conditions or the actual magnitude of the problem. To me, this sounds like a California problem, or at least a dry climate problem. Anywhere it doesn’t rain for weeks or months on end, and where in total it doesn’t rain much, has a lot more time to build up oil and grime versus somewhere like Ohio or Florida where it can rain multiple times a week for a lot of the year. Motorists in such dry climates can get away with bald tires a lot longer, especially if it doesn’t get cold enough to snow, so their traction is great until it gets just a little wet then they’re screwed. That would result in some confirmation bias. “It started raining and my car tires immediately started slipping, but then after a while it wasn’t so bad.” Of course after the initial incident they drove more carefully to prevent it from happening again. Also, excepting brand new asphalt pavement that’s never been rained on, the amount of oil dripping from vehicles and collecting on pavement is pretty minuscule nowadays. Brake dust, rubber flakes, or other grime seems to be pretty minimal too since much of it can blow away in the wind. I’ve never noticed the first flush of water into the storm drains being particularly dirty, but I don’t live in a desert or Mediterranean climate. Is there any measured data out there to corroborate this? Have some state transportation departments done any studies? Like I said, I don’t doubt that it’s technically correct (the best kind of correct), but just how correct is it? ---
**Author:** ![Elmer\_J.Fudd](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/elmer_j.fudd/32/1211_2.png) [@Elmer\_J.Fudd](https://boards.straightdope.com/u/Elmer_J.Fudd)\ **Post date:** [July 22, 2025, 5:22am UTC](https://boards.straightdope.com/t/roads-are-most-slippery-just-after-it-starts-raining/1021159/2 "2025-07-22T05:22:18Z")
Two chestnuts of California driver lore are that the roads will be slipperier than a greased pig after the first rain of the season and that in the intervening months all California drivers will have forgotten how to drive in the rain (i.e. slow down and turn on your headlights.) ---
**Author:** ![robby](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/robby/32/11048_2.png) [@robby](https://boards.straightdope.com/u/robby)\ **Post date:** [July 22, 2025, 5:58am UTC](https://boards.straightdope.com/t/roads-are-most-slippery-just-after-it-starts-raining/1021159/3 "2025-07-22T05:58:00Z")
> [@jjakucyk](#): > > Is there any measured data out there to corroborate this? Have some state transportation departments done any studies? Like I said, I don’t doubt that it’s technically correct (the best kind of correct), but just how correct is it? There are environmental studies to back this up. The first bit of rain and resulting stormwater runoff produces a “first flush” in roadways that is markedly higher in contaminants. Much of these contaminants are due to petroleum byproduct drips and leaks from vehicles. So that first flush contains a lot more suspended oil, etc. as compared to when it’s been raining for a while. (I’m an environmental engineer, and my master’s thesis was on the subject of roadway stormwater runoff.) ---
**Author:** ![scabpicker](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/scabpicker/32/8268_2.png) [@scabpicker](https://boards.straightdope.com/u/scabpicker)\ **Post date:** [July 22, 2025, 6:57am UTC](https://boards.straightdope.com/t/roads-are-most-slippery-just-after-it-starts-raining/1021159/4 "2025-07-22T06:57:52Z")
> [@jjakucyk](#): > > Is there any measured data out there to corroborate this? I know my sister’s first wreck was written off by the insurance company as “summer ice” caused by the first rain in a couple of months. I don’t think the insurance companies dismiss much without at least some data to cite. ---
**Author:** ![Seanette](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/seanette/32/9139_2.png) [@Seanette](https://boards.straightdope.com/u/Seanette)\ **Post date:** [July 22, 2025, 6:59am UTC](https://boards.straightdope.com/t/roads-are-most-slippery-just-after-it-starts-raining/1021159/5 "2025-07-22T06:59:29Z")
> [@Elmer\_J.Fudd](#): > > Two chestnuts of California driver lore are that the roads will be slipperier than a greased pig after the first rain of the season and that in the intervening months all California drivers will have forgotten how to drive in the rain (i.e. slow down and turn on your headlights.) Can they be said to have forgotten what so far as I can tell they never knew? ---
**Author:** ![MrDibble](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/mrdibble/32/114_2.png) [@MrDibble](https://boards.straightdope.com/u/MrDibble)\ **Post date:** [July 22, 2025, 7:29am UTC](https://boards.straightdope.com/t/roads-are-most-slippery-just-after-it-starts-raining/1021159/6 "2025-07-22T07:29:14Z")
Here’s a 2003 study that confirms the effect. > **[The mixed effects of precipitation on traffic crashes - PubMed](https://pubmed.ncbi.nlm.nih.gov/15094418/)** > > The lagged effects of precipitation across days may be explained by the clearing of oil that accumulates on roads during dry periods or by the conditioning of people to drive more safely in wet conditions. Either way, policy interventions that... and an article discussing the study > **[10.31.2003 - Rain is more lethal for drivers after a long dry spell, says new...](https://newsarchive.berkeley.edu/news/media/releases/2003/10/31_rain.shtml)** The takeaway there is that it’s not long dry spells that matter, so the effect would hold even in relatively rainier states. ---
**Author:** ![Mama\_Zappa](https://avatars.discourse-cdn.com/v4/letter/m/71e660/32.png) [@Mama\_Zappa](https://boards.straightdope.com/u/Mama_Zappa)\ **Post date:** [July 22, 2025, 9:12am UTC](https://boards.straightdope.com/t/roads-are-most-slippery-just-after-it-starts-raining/1021159/7 "2025-07-22T09:12:47Z")
I also wonder if it varies by how heavily-trafficked the road is. In an area where it rains once or twice a week, but many thousands of cars drive the same road every day, you’d expect there to be more build up of slippery stuff - while a desert road that gets 3 cars a day might not be as bad. ---
**Author:** ![Spoons](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/spoons/32/3141_2.png) [@Spoons](https://boards.straightdope.com/u/Spoons)\ **Post date:** [July 22, 2025, 9:25am UTC](https://boards.straightdope.com/t/roads-are-most-slippery-just-after-it-starts-raining/1021159/8 "2025-07-22T09:25:43Z")
It’s not just California. It happens everywhere there is asphalt or concrete roads and drivers trying to navigate them. I’m in Canada; we always have to adjust our driving to conditions. Rain, snow, sleet, somehow or other, we get by. ---
**Author:** ![bob\_2](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/bob_2/32/3341_2.png) [@bob\_2](https://boards.straightdope.com/u/bob_2)\ **Post date:** [July 22, 2025, 9:47am UTC](https://boards.straightdope.com/t/roads-are-most-slippery-just-after-it-starts-raining/1021159/9 "2025-07-22T09:47:14Z")
Many years ago, I had a part-time job on Saturdays in London, working as a dispatcher in a garage. I sat in an office, taking calls from people who needed help, the police at a collision, or the AA (we had a contract with them). We normally had three drivers working, fixing breakdowns or towing wrecks in, but if it rained after even a few dry days, they put another driver on as they knew there would be more collisions. ---
**Author:** ![Melbourne](https://avatars.discourse-cdn.com/v4/letter/m/b5e925/32.png) [@Melbourne](https://boards.straightdope.com/u/Melbourne)\ **Post date:** [July 22, 2025, 10:26am UTC](https://boards.straightdope.com/t/roads-are-most-slippery-just-after-it-starts-raining/1021159/10 "2025-07-22T10:26:42Z")
> [@MrDibble](#): > > The takeaway there is that it’s not long dry spells that matter, so the effect would hold even in relatively rainier states. ??? Those are complex the way they are written, but the takeaway is that long dry spells do matter, to the extent that repeated rain is actually safer than occasional rain. FWIW, roads are notably slicker here _after_ the first rain shower. It may be that it takes a while for the surface to soften up. Or, it may be that the road is no worse than any wet road, but when the sun is out, and people are used to dry roads, they aren’t on their best “wet road” behaviour. Or both. ---
**Author:** ![Dewey\_Finn](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/dewey_finn/32/4222_2.png) [@Dewey\_Finn](https://boards.straightdope.com/u/Dewey_Finn)\ **Post date:** [July 22, 2025, 10:37am UTC](https://boards.straightdope.com/t/roads-are-most-slippery-just-after-it-starts-raining/1021159/11 "2025-07-22T10:37:57Z")
This was the subject of the only question I got incorrect when taking the written test to get a driver’s license as a teenager. ---
**Author:** ![MrDibble](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/mrdibble/32/114_2.png) [@MrDibble](https://boards.straightdope.com/u/MrDibble)\ **Post date:** [July 22, 2025, 11:54am UTC](https://boards.straightdope.com/t/roads-are-most-slippery-just-after-it-starts-raining/1021159/12 "2025-07-22T11:54:09Z")
> [@Melbourne](#): > > he takeaway is that long dry spells do matter, to the extent that repeated rain is actually safer than occasional rain. Sorry, that was poorly-worded on my part, I meant that even if there’s a shorter gap (as opposed to months), the effect still shows (but less, as you noted). So it happens even in states that only have a few days-long dry spells, despite the OP’s thinking. I wasn’t trying to say that the length of the dry spell had no correlative effect, although I can see now that it can read that way. ---
**Author:** ![Chronos](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/chronos/32/134_2.png) [@Chronos](https://boards.straightdope.com/u/Chronos)\ **Post date:** [July 22, 2025, 12:17pm UTC](https://boards.straightdope.com/t/roads-are-most-slippery-just-after-it-starts-raining/1021159/13 "2025-07-22T12:17:37Z")
Surely it’d also depend on how hard it’s raining. Sometimes a storm starts suddenly, and you go from no rain at all to pouring in thirty seconds, but sometimes rain starts very gradually, going from nothing to a very thin mist to a slight drizzle to a stronger drizzle to a steady rain to a downpour over the course of hours. And for a very light rain, the water level is often less than the thickness of the texture of the road, leading to almost no effect. ---
**Author:** ![Alessan](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/alessan/32/465_2.png) [@Alessan](https://boards.straightdope.com/u/Alessan)\ **Post date:** [July 22, 2025, 12:48pm UTC](https://boards.straightdope.com/t/roads-are-most-slippery-just-after-it-starts-raining/1021159/14 "2025-07-22T12:48:31Z")
On the other hand, particularly heavy rain would wash the contaminants out faster, wouldn’t it? So the road would be slippery, but only for a relatively short time. ---
**Author:** ![Pleonast](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/pleonast/32/1183_2.png) [@Pleonast](https://boards.straightdope.com/u/Pleonast)\ **Post date:** [July 22, 2025, 1:15pm UTC](https://boards.straightdope.com/t/roads-are-most-slippery-just-after-it-starts-raining/1021159/15 "2025-07-22T13:15:46Z")
> [@Chronos](#): > > And for a very light rain, the water level is often less than the thickness of the texture of the road, leading to almost no effect. Quite the opposite. A very thin layer of water is all that’s needed to create lubrication. See ice skating. A little drizzle on top of months of oily crime creates a surface with little traction. ---
**Author:** ![Chronos](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/chronos/32/134_2.png) [@Chronos](https://boards.straightdope.com/u/Chronos)\ **Post date:** [July 22, 2025, 1:22pm UTC](https://boards.straightdope.com/t/roads-are-most-slippery-just-after-it-starts-raining/1021159/16 "2025-07-22T13:22:37Z")
It doesn’t matter how slippery a surface is, if that’s not the surface that tires are contacting. ---
**Author:** ![LSLGuy](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/lslguy/32/5813_2.png) [@LSLGuy](https://boards.straightdope.com/u/LSLGuy)\ **Post date:** [July 22, 2025, 1:23pm UTC](https://boards.straightdope.com/t/roads-are-most-slippery-just-after-it-starts-raining/1021159/17 "2025-07-22T13:23:48Z")
There are two separate effects at work. One is creating a lubricious layer, the other is creating conditions suitable for hydroplaning. Very thin to microscopic water layers mixed with contaminants promote the first effect, and pavement surface roughness has little effect. As @Chronos said. Deeper water layers (1/8"+) promote hydroplaning and surface roughness is designed particularly to break up that deeper layering. ---
**Author:** ![We\_re\_wolves\_not\_werewolves](https://avatars.discourse-cdn.com/v4/letter/w/838e76/32.png) [@We\_re\_wolves\_not\_werewolves](https://boards.straightdope.com/u/We_re_wolves_not_werewolves)\ **Post date:** [July 22, 2025, 1:37pm UTC](https://boards.straightdope.com/t/roads-are-most-slippery-just-after-it-starts-raining/1021159/18 "2025-07-22T13:37:37Z")
> [@Dewey\_Finn](#): > > This was the subject of the only question I got incorrect when taking the written test to get a driver’s license as a teenager. hah, me too! I remember this exact question ---
**Author:** ![engineer\_comp\_geek](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/engineer_comp_geek/32/504_2.png) [@engineer\_comp\_geek](https://boards.straightdope.com/u/engineer_comp_geek)\ **Post date:** [July 23, 2025, 12:36am UTC](https://boards.straightdope.com/t/roads-are-most-slippery-just-after-it-starts-raining/1021159/19 "2025-07-23T00:36:12Z")
> [@jjakucyk](#): > > To me, this sounds like a California problem I have personally experienced it in West Virginia and in Pennsylvania, so it’s not just a California problem. You do tend to see it only after a bit of a dry spell, but you don’t need a California wildfire seasons type of dry spell. You just need a few weeks of dry weather, which you get just about everywhere. Except this year. Pennsylvania has had rain at least a few days out of every week for the entire spring/summer this year. But in normal years we usually get a few weeks of dry weather every summer.