Transportation engineering is a branch of civil engineering that is involved in the planning, design, operation, and maintenance of safe and efficient transportation systems. These systems include roadways, railways, waterways, and intermodal operations. Typically, the demand is the amount of traffic (people, cars, railcars, barges) that is expected to use a particular transportation facility, while the supply is the quantity and type of infrastructure components (roadways, bridges, pavements, etc.). These systems are typically large and expensive. There are a number of attributes of transportation engineering that affect the types of statistical theory that are used in the profession. One important aspect of transportation engineering is that the transportation engineer is not only interested in the infrastructure (e.g., bridges, rails, etc.) and the individual units (cars, trucks, railcars) that use the infrastructure, but also the user.