Recently we've discussed various things about how the tyres and the driver achieve some semblance of traction and control (they're available online in the Life & Style section, just click on my name at the top of one of my earlier stories), so it's probably time we discussed the road surface.
The surface is only the topmost of many layers that make up a road of any kind. Just its skin you could say. The first layer is actually there for drainage. Often that's rocks and coarse gravel put there to be a blind drain.
The surface itself can take many forms. It can be in a wide variety of conditions, and then there's the influence of friction modifiers like water, ice, oil or anything else that may reduce the grip. But we'll get to those.
First, I want you to observe how the road can be placed at a huge variety of angles in relation to the earth. Some of these angles are favourable to corner grip, but on public roads, they're often not. The angle can also influence which way the car wants to go in an emergency stop, so think about that as well.
In an effort to drain the water off, which is the main priority, the surface needs to be tilted away from level a little bit.
In some places this tilt can be in line with the direction the road is headed, but sooner or later the water needs to be sent off to the side. On single carriageways, that's typically to the driver's left, while on multi-lane carriageways it can be to the left of the left lanes but to the right of the right lanes.
In corners it could be tilted in any way, and it can change through the corner. On the F3 motorway between Sydney and Newcastle for example, it was decades before the off-camber tilt (especially worrisome on wet downhill corners) was rectified so that it banked a little more like a race track instead.
There's a reason why oval racing - still astoundingly-popular in north America - banks their corners so that the inside of the track is substantially lower than the outside. Centripetal force (yeah, we're getting back to high school physics again), is in this case the banked track itself helping counteract the moving object's inherent tendency to go straight. And so, the opposite is true for any spot that's off-camber; even less traction is available than if the road was simply level.
As for the surface itself, there are different types of sealed and unsealed roads. The latter could be hard-packed dirt, or loose dirt, or sand, or gravel, or grown-over with grass and weeds, or something else, and each of those is vastly different in grip (and in feel).
Oh, and a bridge on an old backroad with a timber deck is also something you may find, and they're as slippery as algae-covered rocks when they get at all damp so definitely proceed with caution, even on foot.
Different sealed surfaces offer different levels of grip too, especially in the wet. The goop used to seal cracks is something every motorcyclist is told to watch out for. Paint can be slippery to, and wet metal grates always are. Then there's concrete, which tends to be smoother than tarmac, and therefore less water is needed to create aquaplaning (which is the tyre skimming the surface of the water for a moment, offering literally zero grip).
Even the age of the tarmac has an influence. Fresh tarmac has a slightly more coarse and textured surface. That aids traction a bit, but as race teams have discovered it also wears tyres faster. Race drivers also look forward to a track rubbering up over a race weekend, especially if it's a temporary street circuit, because race-rubber that's effectively been moulded onto the racing line actually offers more grip than bare tarmac.
We don't get that on the street though, so instead we find dirt gets deposited in the low points and the peaks that initially grab tyres nicely get slowly worn off, making the surface smoother. Consequently, old tarmac is also quite slippery when wet.
Plus, don't forget that in some places ice can easily form on cold mornings and be very difficult to see (so it gets called black ice).
This is all assuming the road is otherwise in good condition. Ruts and other bumps also cause individual tyres to have their grip levels negatively affected, making things even trickier in an emergency.
Sam Hollier is an ACM journalist and a motoring fanatic who builds cars in his shed in his spare time.