The standard municipal sidewalk is four feet wide and is generally poured in twelve to fifteen foot slabs. Control joints are placed at four foot intervals in shorter slabs and five foot in the longer.
At the ends of the slabs are expansion joints made of a fibrous material and tar. this allows the sidewalk to move in severe temperature changes. Figuring a standard temperature of 68F, the slabs will either expand or contract from this design figure.
In Central Ohio the lows can range from a -20F to a high of around 90F. You can see that a lot of movement has to be taken up by the expansion joints. These help to reduce thermal cracking of the slabs.