Abstract An activity model in an urban area is investigated to form the basis for the development of a demand model for transportation planning. As suggested by Chapin, a transportationactivity is the off-spring of the evolutionary processes involving motivation-choice-activity and optimization processes. Following his hypothesis about activities, we have developed an activity model which might be applicable to transportation planning. Using the analogy of transportation systems on queueing models, a representation of transportation and activity systems by serial queues is investigated in urban and surburban areas. In this model time required for transportation and various activities are assumed as the key factors to determine the number of trips for users within the area. A newly conceptualized cost-effectiveness function is used to obtain optimal trip patterns of users in these areas. Maximizing this function, an optimal visiting rate is calculated for any specification of transportation and activity time. Some realistic examples are depicted to illustrate this model and good insights into trip patterns are obtained for a variety of trips from 20 min to many hr in duration.