Some products have elastic demands, while other goods have inelastic demands. Typically, the derived demand for transport follows a similar pattern to the elasticity of the final good, but not always. The share of transportation costs in the final price of the goods may be large or small, regardless of the product's own demand elasticity, and this can affect the elasticity of the derived demand for transport.

(a) With the aid of an appropriate economic model(s) explain the conditions in which the derived demand for transport would be elastic or inelastic.

(b) Explain why an increase in the number of transportation substitutes would affect the elasticities of the derived demands for transport, regardless of the product carried.