This results from the immediate consumption of a road, if it is to be considered as a good, preventing the possibility of storing the good for periods of greater demand. The demand placed upon road transport is therefore affected by three main factors; the price of road transport, the price of alternative transport services, and individual income. However the true cost of road transportation is hard to quantify as there are many external costs not considered by an individual when calculating the marginal cost and benefit ratio before undertaking a journey. These externalities include the cost of congestion, especially during peak periods adding to the cost of fuel and time to the journey, air pollution and its resulting effects upon human health and the environment, noise pollution effecting property value in areas surrounding busy roads, and the costs of traffic law enforcement.