Why don't they make trucks aerodynamic from the sides?

[Mr Macaw]

A lot of trucks (eg a Parcel Force truck) doesn't need the full width at the top. Can't they make them curved like cars so high winds don't blow them over?

[News]

In message , Mr Macaw writes

OK, I'll accept this as a genuine question. Cannot speak for PF, but I know RM use the full space inside. Mail is loaded within large wire trolleys (Yorkies) which are loaded using a tail hoist, and the Yorkies fill the complete vehicle, width and height. No wasted space so no room for roof curves.

[Mr Macaw]

OK, I've picked a bad example, but do all trucks get loaded right to the roof?

[newshound]

It's not the curved tops that stop winds blowing cars over, it is the fact that the height to track ratio is so much lower. An aerodynamic "feature" that worked for one wind direction would not work for another, and would add drag.