What is road friction

We've been producing friction meters to be installed in cars for fairly long now, close to 20 years or so. Idea itself is fairly simple; you measure the deceleration during braking and from that you can calculate relative friction of the road. First models interfaced with car systems directly, and some newer models use accelerometer to measure deceleration.

Lately I've been playing with slope compensation as well. In addition of direct friction (which is affected by extra acceleration or deceleration caused by gravity) the slope is determined too and that is used to compensate the gravity-modified friction reading. Relatively simple math, really.

But what is this used for, then?

The winter season in Finland is in average maybe 5 to 6 months. I say 'season', because weather varies a lot, and actual slipperiness of road may change a lot in just a few hours, especially at autumn and spring. Proper winter tires are absolutely essential, and in fact mandatory.

And since it is winter, there will be snow. Amount varies by latitude, but generally amount varies from "enough" to "way too much". And despite this Helsinki-Vantaa international airport practically never closes due to snow - and if it does, it is for maximum of hour or so during the worst storms. This should already tell you something about how seriously wintertime maintenance is taken here. Same applies to our roads - snow plows take off (to work) after just few centimetres of snow and they keep the roads open, every day, all day. And yes, of course there are some periods after heavy snowfall when roads may be in bad shape before they get cleaned, but that is only for few hours typically.

Road here may seem slippery, but if I remember correctly, this specific section actually has 100km/h speed limit even in winter and you can actually drive at that speed relatively safely most of the time. I don't recommend that however if you have bad tires or have never driven in winter.

Like you see above it's impossible to can't get all snow and ice off the road (that shine on asphalt is ice), there always is some remaining. Fine gravel and salt are used to slipperiest places, but how you determine where it is slippery? Keeping the roads in pristine condition isn't exactly cheap.

That's where friction meters come into play. They are used widely to track road condition by measuring road condition so persons responsible for the maintenance know almost immediately when something needs to be done. Determination is done by having all friction meters calibrated to measure same reading at same conditions and comparing these readings about nationwide standards. While experienced people know the road condition instinctively, measurement makes the maintenance easier by giving quantifiable and documentable readings on which the decisions can be based on.

While this might seem a lot of effort, it actually makes the traffic work very well. Finland is large, sparsely populated country, and lot of goods and people travel by roads. When roads are known to be in good, safe condition traffic can move quickly, easily and safely around, year round. There just are no "snow days" in Finland.

Want to learn even more? Winter road congress is coming up again, on February at Tampere. We'll be there too.