Abstract
Recently two a priori approaches have suggested that modal-split and associated economic-demand functions may not exhibit mathematically continuous properties. This paper presents the results of some empirical findings which support the essential conclusions of these theories. In particular it is estimated that the effect of petrol price changes on weekend traffic levels in London depends nonlinearly on the size of the change, its direction, and on previous petrol price changes, owing to a strong habit effect. On average there is a habit threshold approximately equivalent to a sevenpence change in the price of petrol (1974 prices). Some policy implications of this empirical relationship are explored.
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