THEORY OF LAND TRANSPORT
The theory of land transport in the pre-industrial age is limited to a consideration of walking, running, carrying, and carting. Within this restricted field, however, is a body of analysis that provides the basis for the design and construction of virtually all pre-industrial, and many modern vehicles. This theory, of course, was not understood at the time these vehicles were developed and such development necessarily relied on trial and error methods. Techniques that worked survived and were used repeatedly until the best designs for the current conditions emerged. Darwin's theory of natural selection and the survival of the fittest applies equally well to mechanisms as to animals and plants. The analysis presented in this chapter confirms the validity of such empirical methods and provides a deeper understanding of why vehicles developed the way they did, and why their performance was limited.
We begin by considering the energy required in walking and running. This was partially treated in Chapter Five but here a theoretical treatment that fits the measured performance, is presented. Speed depends on step length and step frequency and the transition from walking to running occurs because it is no longer possible to increase step length without both feet leaving the ground. The theory of walking in crowds is described and explains why the density of a crowd affects the average walking speed and eventually brings it to a halt. It also explains why soldiers march in ordered columns. The theory is equally valid for vehicles on the highway and does much to explain congestion in towns and cities, which was as much a problem in ancient and medieval times as is it today.
The force required to move two-wheeled and four-wheeled vehicles is examined to reveal the optimum geometry that minimises the load placed on the prime mover, usually a draught animal. It turns out that single-axle vehicles travelling over hard surfaces should have