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?
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.)
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.)
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.
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.
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.
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.
??? 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.
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.
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.
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.
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.
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.
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.