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.
- That roads are actually more slippery at first rain because of mixing with oils and dirt.
- That driving is actually more dangerous at this time.
- 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