Plowing cul-de-sacs is one the most challenging operations during a snow storm. Most properties on cul-de-sacs have frontages which are narrower than lots on a straight street. Therefore, snow must be pushed into a smaller space. Cul-de-sacs without islands contain even more area which needs to be plowed and, as a result, even more snow has to be moved to the side.
It is impossible to have the truck push snow into the center of cul-de-sacs for several reasons. Trucks are not designed or built to be articulated enough to push snow to the center; this applies to cul-de-sacs with or without islands. Centrifugal force causes the snow to move off the plow toward the outside of the circle. Also the center of most cul-de-sacs is an inappropriate spot for snow, not to mention icing problems that would result from melting snow in the center of cul-de-sacs.
Drivers do their best to avoid plowing cul-de-sac driveways in with excessive snow. But this requires finding open spaces between driveways in the cul-de-sac to place the snow. Depending on driveway locations, this may result in large mounds of snow in only one or a few spots in some cul-de-sacs.