We linked the concept of friction to traction on the road. In this case our roads were two foot long lengths of 2 x 4's and our vehicles were plastic containers weighted down with sugar. Our control piece was the plain board, a second board was coated with oil -- I likened it to the oil slicks raised when it begins to rain. The final board was coated with ice to simulate winter driving. I found the best way to prepare the board was to lay it flat in the freezer the day before and mist it with a spray bottle several times a day.
The kids placed their "vehicles" at one end of the board and slowly began lifting them. They stopped as soon as the containers started slipping and measured the height of the board and the angle of the slope.
I have to say I was surprised at the results (I was a bad teacher and didn't follow my own advice to always try the experiment before doing it with the kids). The icy board did have the least friction, but the oil-coated board actually required greater height and slope than the plain wood. In retrospect, I wonder if the rain actually lifts the oil off the road surface and perhaps I should have wet the board down before (or after) applying the oil. If anyone tries this experiment, please let me know how this turns out.