Why are roads sometimes extremely slippery after rain fall?

Question

We've had 5cm of rain in the last week. Yesterday and early this morning it rained. During the morning commute the roads were still wet, but not it was not actively raining. The temperature was $7^{\circ}$Celcius.

The road (asphalt) was as slippery as black ice. I've driven this road many times when it was wet from rain and I never noticed it being so slippery. I was not hydroplaning. I saw a vehicle spin out. On a ramp that I can normally drive 65km/h on, I could only drive 30km/h without fish-tailing. Something felt very different about today's wet pavement compared to other days. In fact, I drove the same road yesterday (while it was raining) and it did not seem so slippery. When I got to work, I even checked the tread on my tires, and they looked fine.

I've read that the oils from cars can make the roads especially slippery on the first few rains. We've had 10cm of rain in the last month, so I don't think this is the case.

Question:

1. What could cause the roads to be especially slippery this morning? It felt like something 'special' had happened because wet roads are never that slippery after or during a rain.

Answer 1

Usually roads become slippery after a first rainfall of the season because of all the dust and oil particles that have not been washed away and they are mixed with the moisture. Basically they create a slightly more sticky film (compared to rainwater) that acts as a lubricant between the road and the tire.

(Any rain Harvesting system uses a First flush diverters to divert and capture the most contaminated, sediment-laden water that washes into your pipes from the roof during the first initial millimetres of rain.)

However from what you described this:

• was not a first rainfall after quite sometime,
• It rained quite a lot (the 5 cm of rain is quite a lot)
• you weren't driving shortly after the rain started.

so I'm not convinced that is the root cause.

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Answer 2

As you mentioned bitumen, tire rubber particles, fine dust, and oil can cause a roud to be slippery after a dry spell.

But in your case it's not that. it could be a caravan of old trucks crossing the road earlier, have left a perfect mix of oil and rubber, or their load could have been constraction silty/gypsum material which has blown over the road due to use of old tarp to cover them.

Answer 3

Hydroplaning - By definition, hydroplaning is when a vehicle starts sliding uncontrollably due to the tires encountering more water than the treads can displace. When driving at high speeds over wet pavement, the water begins to push the front tires off the ground slightly, creating a thin film between the tire and the road. That film, and the resulting separation of the road and tire, causes the driver to lose control of the car and possibly end up in a crash.

Note the words "thin-film" - the thinner the medium causing the separation, the more difficult to control. Consider it is more likely to lose footing on an extra thin plastic sheet placed on the floor compared to a thicker sheeting. Also, the thicker water layer, while can cause instability, it offers more resistance in the front face of the tire, which the thin film can't do.

Another reason is the human perception - during heavy rainfall, we see the wet pavement and are prepared for the slippery condition; for the thin film formed in the early morning, the wetness of the pavement isn't that obvious to be recognized until one starts to speed up or make turns.

The above are my two cents.