Vehicle Traction
A vehicle's effective traction wheel area is the sum of each wheel's area multiplied by the relative move cost of the terrain it is on, with pavement being the norm.
A relative move's cost depends upon the wheel type.
A vehicle's acceleration and safe/top speeds are calculated by its current k_traction, which is
wheel_traction_area / ( ( 1 - wheel_traction_area / sum of wheel area ) * vehicle mass )
Basically, contact area is how much of the tire is in contact with the ground and affects vehicle ground pressure, but it is complicated because two wheels with the same contact area can have vastly different designs and handle terrain differently. For example, a railroad wheel and an off-road wheel might have the same contact area, but one is a flanged rigid metal disk that has terrible performance on anything but a railroad rail, and the other is a knobbed soft rubber tire that can actually apply that contact area across a wider variety of terrain.
k_traction, like all k constants in CDDA, is legacy code that can be difficult to work with. It has been adapted, rather than rewritten, to work with the vehicle physics model improvements. The individual components of k_traction mostly make sense - we want to calculate some kind of terrain adjusted ground pressure and use that to influence how efficiently a vehicle's drive wheels can actually put down force.