A theory of the decision-making behavior of urban households in allocating the use of their automobiles among essential travel purposes is developed. This theory begins to assess interdependence of household members with respect to their tripmaking behavior. The nature of trips to be made is predetermined by fixing the extent to which essential purposes must be served. Also, limits on household automobile availability and the importance of trip timing are recognized. Mathematical programs are developed based upon the assumption that households attempt to minimize travel costs required to achieve essential purposes subject to constraints on automobile availability.