The Lighthill-Whitham-Richards kinematic wave traffic flow model was extended to describe traffic with different types of vehicles, in which all types of vehicles are completely mixed and travel at the same group velocity. A study of such a model with two vehicle classes (e.g., passenger cars and trucks) showed that when both classes of traffic have identical free-flow speeds, the model (a) satisfies the first-in-first-out rule, (b) is anisotropic, and (c) has the usual shock and expansion waves and a family of contact waves. Different compositions of vehicle classes in this model propagate along contact waves. Such models can be used to study traffic evolution on long crowded highways where low-performance vehicles entrap high-performance ones.