The shape of the stone unit determines how the stone surface responds under load and the way in which the load will be resisted.  Certain shapes of units are better suited to particular forms of construction.  While they all can be appropriate for pedestrian use, driveway and city streets have tighter parameters to pay attention to.

These terms are used throughout the paver industry:

Aspect Ratio – The overall length of a paver divided by its thickness of the paver unit.

Plan Ratio – The overall Length of a paver divided by its width.

Plan Area – The Length times the Width of the paver.

New  – Modular Ratio – The relationship between the depth of a unit and its plan area.

 Aspect ratio diagram

  SCOTS – NATURAL STONE SURFACING, Good Practice Guide – 2nd Edition December 2004

There is an important relationship between the depth of a natural stone element and its plan area.  This is often termed the modular ratio of the paving unit.  The modular ratio determines how the unit will react under load in two ways.  Firstly, it determines if an individual unit of a given strength will fracture under a directly applied load.  Secondly, it influences how the paving surface will resist loading and how the surface will react in different ways under load depending on the ratio of their depth to plan area.

 In Surfaces of cubes, setts, and blocks, the resistance is predominately through shear in the jointing

In tiles and flagging, the resistance is predominately from the bedding.

Porphyry cubes, from Milestone Imports, in addition to being the most popular paving material, are a stable and sustainable vehicular paving choice.