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From DriversEd.com: Rain:

Slow down at the first sign of rain, drizzle, or snow on the road. This is when many road surfaces are most slippery because moisture mixes with oil and dust that has not been washed away.

Is it true that roads are more slippery when rain starts compared to after it has been raining for a while. I would have guessed that after puddles can build up everything gets dangerous. There are a few different claims that I want to investigate.

  1. That roads are actually more slippery at first rain because of mixing with oils and dirt.
  2. That driving is actually more dangerous at this time.
  3. That the reason that driving is more dangerous at this time is because it is more slippery (and not because people are more careless because it doesn't look as bad)

Claim 2 is the main question, 3 might not have sufficient research to be answered properly.


Here is a link that makes all three claims explicitly, since there was some contention about that. Texas Insurance: The Most Dangerous Time to Drive when it Rains

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Yes, and so much so that it can significantly affect accident rates.

A good study exploring this phenomenon is The mixed effects of precipitation on traffic crashes by Daniel Eisenberg.

...analysis shows that the risk imposed by precipitation increases dramatically as the time since last precipitation increases. For example, 1 cm of precipitation increases the fatal crash rate for a state–day by about 3% if exactly 2 days have passed since the last precipitation and by about 9% if more than 20 days have passed. This basic pattern holds for non-fatal crashes as well.

Which supports the notion that :

That driving is actually more dangerous at this time.

is true.

As to the 1st and 3rd statements, the paper suggests that the oil/dirt build up may be a factor:

The lagged effects of precipitation across days may be explained by the clearing of oil that accumulates on roads during dry periods

but offers no analysis to support this directly. The possibility that the problem exists between steering wheel and chair is also acknowledged:

or by the conditioning of people to drive more safely in wet conditions.

While not exactly scientific (anecdotes are not data!) my own experience suggests there is likely a combination of the two at play.

  • 1
    I would guess that a lot of it relates to human behaviour more so than the actual condition of the road surface. It would be interesting to see some research aimed towards road surface conditions over time during precipitation. This study seems to just be analyzing data and then assuming that this is one of the causes; but it's not clear if that's a significant reason. It seems like the numbers could be explained similarly by assuming that drivers just grow used to better or worse road conditions over time, so it changes for the worse after periods of low precipitation. – JMac Feb 6 at 12:18
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    Welcome to Skeptics! Can you please quote a little from the study to show us where it says this and to protect from link rot? – Oddthinking Feb 6 at 14:36
  • @JMac If it were all attributable to human behavior, I wouldn't expect to see the lagged correlation between precipitation and accident rates in dry conditions. Rain yesterday is correlated with fewer accidents today, regardless of whether it's raining today or not. I agree there's probably some component of "getting used to" several days of bad weather that improves driver behaviors, but I wouldn't think that effect would persist when the weather clears up. Also, it stands to reason that not everyone drove yesterday, so there can be no adjustment of behaviors for those individuals. – Nuclear Wang Feb 6 at 14:42

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