When is rain covered road most slippery?

. I have found through experience that the roads are more slippery when it has rained a little and the road is not completely covered by water. I believe this because I use to ride a motorcycle. I think their are more wrecks just after it starts raining because people think that the road is still safe to drive fast. Some people do not adjust their speed for the condition of the road until the roads are completely wet.