Some road surfaces are more slippery than others when wet.

Yeah because when people drive on the roads oil can drip from the vehicles and get on the pavement, so when it rains there can be roads that have more oil on them than others..

Period is the time it takes for one “cycle” to complete. in the parent sine function, period is 2*pi since sin(0) = 0, sin(pi/2) = 1, sin(pi) = 0, sin(3pi/2) = -1, and sin(2pi) = 0. at that point, the sequence repeats itself. i’m not sure if your graph is similar to the sine function since you didn’t provide a picture, but you can find the period by looking at a graph.